What Natha said. To get to the sheets you create a new game and there is a dropdown box to select the sheet you want. The 5e sheet you want is the one that says "by Roll20". Once you have a game, you load it up and on the top right there's a line of buttons, one of which is Journal. Click the Journal button and then click the Add button. That will create a new character sheet for you. You might also benefit from checking out the tutorial with can be found under Games at the top of the roll20 page, it's the bottom option. I would also recommend watching a few youtube tutorial videos lots of them are very clear and helpful and will help you get started.