Hi Sir Dolphin and welcome to Roll20. I would highly recommend the Virtual TableTop Tutorial that shows you how to use the various parts of the software. Additionally, starting your own test game to play with the controls is an excellent way of getting used to the software, and you really can't break anything I promise! As for sheets specifically, you select one of the community sheets , such as the 5e OGL, when setting up a game (or no sheet at all if you just want to use the base VTT features). Then, when you log into the game you need to create journals (the tutorial will help with that) and assign them to people to use. The wiki is also a treasure trove of information for new users.