It's an awesome time to be into PnP with Roll20. 5th edition is probably my favorite thus far out of all the systems I played (Including non D&D).
All the classes are superb (except Ranger Beast-Master).
If you choose to DM, here's some great resources, if you're unaware of them.
Reddit list of all official items and some tools.
Bunch of free official WoTC stuff in there. The Unearthed Arcana series is awesome for some extra free content, that is official.
Point Buy Calculator
Great resource for your players.
Great Site to purchase homebrew and WoTC extra additions
Has new archetypes, Classes, Backgrounds, Feats, also has a bunch of adventures.
Adventures are great if your party of players gets off-track and you want to toss them some quick content before you wrap your brain about how you're going to get them there.
Random Generators and other trinkets
This site is great for putting together random dungeons, generating a load of fantasy names or NPCs. Also offers some random quest generators and other tools.
Spell Cards
Great player resource, to help them keep track of their spells. Or for DMs keeping track of spells for monsters. Allows you to customize your 'deck' and save it for reference.
Small tips if you're new to DMing.
Since you play here, you're already practicing improv, which will be your
greatest asset as a DM (Trust me on this one)
If you have trouble fleshing out details on the fly through improv out loud, think about moving the sessions into typing in-character, and leaving voice chat for OOC. (I have done this, and it has greatly improved the interest of my players. They're also more likely to take a moment to think of consequences and take their characters much more seriously. I do handle combat in voice only however, for pace reasons)
I found using the variant spell cost system (Mana-pool instead of spell-slots) will greatly speed up your gameplay, and be less frustrating for you the DM, and your players. It also allows you to keep easier track of monsters if you have multiple spell-casting ones.
Definitely use POINT BUY, for character building. If you're new to DMing you may have trouble balancing encounters around players whom did 4d6(Drop the lowest) for stats.
This also prevents players whom are frustrated that their peers tend to become the lime-light.
Make a plot-outline, and try to follow that. Don't make quest lines that are very detailed, or filled with massive amounts of lore tid-bits. Because 99 times out of 100, your players will decimate those linear quest-lines into the realms of hell. They also will likely avoid any exposition dumps, and tell the wise NPC to shove it.
Giving the appearance of choice, (AKA Rail-roading). Sometimes you need your players to enter a dungeon, or other interior area. On your map,
don't list its location. This will prevent your players from taking a 180 and avoiding it (you'll lose sleep if they do this).
Instead, the next area they're traveling to, try to incorporate it into there. So maybe a town will have a crazed homeless guy rambling about the nearby crypts. Or maybe they'll come across it when they enter the forest to rescue the kidnapped daughter of a farmer (She may even be in there now! PLOT REASONS!). Your players will be super grateful that you
planned this whole dungeon just for them to find in a specific location. Unbeknownst to them, it could have been anywhere.
Think about breaking your game into story-archs. Rather than having an epic level 1 to 20 story line of chasing the bad guy, think about staggering the campaign, into mini events. Also after each campaign story, think about giving them a long down-time (A couple years) along with some property so they can build their characters around a family, or guild they may wish to take part in. This will make them feel more a part of the world, and give you plot to exploit.
As an example,
Level 1-5 ; The adventurers uncover the plot of a necromancer trying to destroy the village by opening a gate to the Shadow-realm
Level 6-10 ; Known as Heroes now, a local Lord invites them to a feast. They are convinced that the cult of the Necromancer has much deeper ties in the Kingdom than thought. They must find the leader and destroy him/her.
Level 11-15 ; The Leader is destroyed but, the gate to the shadow realm has opened! The group must uncover the ancient artifacts in order to close the gate, and track down the evil giant monsters that escaped. They return the artifacts to the King after closing the gate.
Level 16-20 ; The Heroes have become legends, having saved the entire realm. It's time to retire- except the King was the avatar of Asmodeus in disguise! The Heroes band together and become the council of the kingdom to lead the armies against the fiends that now invade all of the realm. In the end they will kill Asmodeus himself!
Level 20+ ; Roll new toons and keep their Heroes as legends of the past, that influence your new campaign. (They'll love this.)
Advice specifically for Roll20
If you choose to make character sheets for each of your monsters and your character sheets are "5th Edition (Community Contributed)" Here's some macros that you can utilize to automatically roll each tokens Action(Attack) without opening their character sheet.
Make sure your monsters are on the 'Monster version' of character sheets, by pressing the top left button within the sheet.
Action 1 (Attack Macro)
@{Selected|npc_output_option} &{template:5eDefault} {{npcaction=1}} {{title=@{Selected|npc_action_name1}}} {{subheader=@{Selected|character_name}}} {{subheaderright=@{Selected|npc_action_type1}}} {{npc_action_description=Attacks @{target|token_name} @{Selected|npc_action_description1} @{Selected|npc_action_description1}}} {{npc_action_effect= Damage @{Selected|npc_action_effect1}}}
Action 2 (Attack Macro)
@{Selected|npc_output_option} &{template:5eDefault} {{npcaction=2}} {{title=@{Selected|npc_action_name2}}} {{subheader=@{Selected|character_name}}} {{subheaderright=@{Selected|npc_action_type2}}} {{npc_action_description=Attacks @{target|token_name} @{Selected|npc_action_description2} @{Selected|npc_action_description2}}} {{npc_action_effect= Damage @{Selected|npc_action_effect2}}}
Action 3 (Attack Macro)
@{Selected|npc_output_option} &{template:5eDefault} {{npcaction=3}} {{title=@{Selected|npc_action_name3}}} {{subheader=@{Selected|character_name}}} {{subheaderright=@{Selected|npc_action_type3}}} {{npc_action_description=Attacks @{target|token_name} @{Selected|npc_action_description3} @{Selected|npc_action_description3}}} {{npc_action_effect= Damage @{Selected|npc_action_effect3}}}
This will allow you to type in a fancy gray text box (Great for typing in your NPC convos,emotes, and background information)
This little macro will allow you to post an image-url in the chat-log, instead of having to upload it onto the roll20 screen for players to see. URL must have image file names at end IE '.PNG'
This macro will show you the traits of your monsters, once their token is connected to a character sheet. (IE; Resistances, Condition immunities, weaknesses, Racial abilities)
@{selected|npc_output_option} &{template:5eDefault} {{title=NPC Traits}} {{npctraits=1}} {{subheader=@{selected|character_name}}} {{npc_damage_resistance=@{selected|npc_damage_resistance}}} {{npc_damage_vulnerability=@{selected|npc_damage_vulnerability}}} {{npc_damage_immunity=@{selected|npc_damage_immunity}}} {{npc_condition_immunity=@{selected|npc_conditiona_immunity}}} {{npc_traits=@{selected|npc_traits}}}
Utilizing these macros will allow you to GREATLY speed up your combat as the DM. This will keep your players more focused on the game rather than their phones, since the round will rotate back to them much faster.