What's needed to start running a campaign on Roll20?

So I'm wanting to start a D&D 5e published campaign using Roll20 and could use some help. Obviously, I need to purchase one of the products like Lost Mines of Phandelver. Do I also need to purchase the Roll20 version of the Players Handbook so my players can make characters? Will they have access to the rules when I invite them to the game? Do I need to purchase the Monster Manual to use monsters or do the campaigns come with all the stats?

It depends on how much you want done for you. At its most basic Roll20 requires you to purcahse absolutely nothing. You could use the hard copy of the adventure if you have it sitting on your bookshelf and manually import the maps and NPCs (tokens and stats) into Roll20. I did this for running an Iron Gods Pathfinder campaign. Buying the published module will get you all the NPCs set up and ready to go as long as you are using a supported sheet, or a sheet that will convert them for you (Obviously the 5e by Roll20, but I believe the shaped sheet also can handle the modules). In addition the module comes with all the information from the book transferred into Roll20 and available in the Journal. As for player characters (or custom NPCs), all character sheets on Roll20 work just like a paper character sheet where you (or your players) can enter their character's stats and abilities in them manually. Or you can use the character mancer with any compendium expansions you've purchased to allow your players to use a wizard to create their characters. TL;DR is, Roll20 can be as automated or as manual as you want.

At a minimum, you bring knowledge of the system (beginner or experienced) you wish to use, any adventure you wish to use (homebrewed, purchased on roll20, or in your hands), and players. As Scott mentions above, it all depends on how much you want done for you vs how much you want to do yourself. You could start a campaign with nothing but blank sheets and your imagination but that would not be recommended for beginning GMs (but it is doable as that is how I got started in the hobby back in the 80's). Good luck and if you have questions, just post it and people will try to help you by answering or pointing you to the information for you to read.

Do I also need to purchase the Roll20 version of the Players Handbook so my players can make characters? Your players can fill in a character sheet just like they would in real life without you buying the PHB, they can use their own PHB or the basic rules or whatever. Players can even use the charactmancer to an extent, but not all the options will be there and you can't just drag a spell to your sheet. Only the free to use SRD stuff will be available. So the PHB is great to have, but not a must buy or anything unless you want the extra options. Will they have access to the rules when I invite them to the game? Yes, everyone will have access to the SRD and if you buy the PHB they'll have access to that while actively in your game, but not outside of it. Do I need to purchase the Monster Manual to use monsters or do the campaigns come with all the stats? If you purchase the Phandelver module it will come with every monster you need already pre-made and placed on the maps in the correct places. This is one of the time-savers that makes the modules worth it generally. If you want to go beyond those monsters then you can drag what's in the SRD in to your game but it won't have any artwork or a token. Buying the MM gives you access to all the monsters in there and you can just drag them in to your game. Hope that all helps.

Thanks for the replies guys and Spren thank you so much for that answer and I applaud you on the formatting as well. That answers everything I wanted to know. Exactly what I needed to know. Spren said: Do I also need to purchase the Roll20 version of the Players Handbook so my players can make characters? Your players can fill in a character sheet just like they would in real life without you buying the PHB, they can use their own PHB or the basic rules or whatever. Players can even use the charactmancer to an extent, but not all the options will be there and you can't just drag a spell to your sheet. Only the free to use SRD stuff will be available. So the PHB is great to have, but not a must buy or anything unless you want the extra options. Will they have access to the rules when I invite them to the game? Yes, everyone will have access to the SRD and if you buy the PHB they'll have access to that while actively in your game, but not outside of it. Do I need to purchase the Monster Manual to use monsters or do the campaigns come with all the stats? If you purchase the Phandelver module it will come with every monster you need already pre-made and placed on the maps in the correct places. This is one of the time-savers that makes the modules worth it generally. If you want to go beyond those monsters then you can drag what's in the SRD in to your game but it won't have any artwork or a token. Buying the MM gives you access to all the monsters in there and you can just drag them in to your game. Hope that all helps.