There are several ways. The GM can buy Maps or Map Tiles in the Roll20 Marketplace (link at top of page). This makes it easy. The GM can go to the Art search tab, in your game, and search for free maps there. Drag it onto the Map Layer and resize it with the corner-handles. You can also make your own maps on your computer or find them online, or draw a map and take a picture, whatever you do that leads to a JPG format map on your computer -- then the GM can go to "Upload" within the game, and upload the map JPG to their Roll20 account, and then place it on the tabletop. Note you can also play Roleplaying Games without any map, and it is really fun! It is sometimes called "Theater of the mind" or just using narratives and descriptions, and imagination --- ditching the map and figurines. If you're new this is what I recommend for starting into the game anyway, at least consider running part of your game session "off map". Roll20 has the advantage that you can use the Art tab to bring photographic pictures or illustrations onto the tabletop --- say, a picture of some dark and evil forest, rather than a map view from the top-down.