You don't need to spend any of the money to get started on Roll20 by joining a D&D game. Use 20-30-40 to buy a Player's Handbook for D&D, the current version is 5E "5th Edition", the starting book is Player's Handbook. Start with a Player's Handbook of D&D. Technically you could buy that book on Roll20, or other digital places. But gosh it's a really nice hardback book, unless you find reading easier on a device, I'd say get the PHB hardback book. U can order that book from Amazon, or, if possible, find a friendly local game store ("FLGS") in your town. If you find the store in your town, you can look over the book on their shelf, decide to buy it, and probably meet some people there in person too. You don't have to read or own the Player's Handbook, but it would be my advice best place to start, at the same time as looking for a game to join as a beginner on Roll20.