Hey there as stated already you can create your own game and export/import characters for ease, but yes this does somewhat require you to know the sheet they will be using (some sheets convert information so you may be ok even if its a different sheet) and that the game creator has turned on importing, if they haven't you'll either have to go through and input all the info into a blank sheet in their game or ask them to turn it on so you can import it (I've never had a GM refuse this so imagine your chances are good). Beyond these options there are services like Myth-weavers.com that will allow you to create and save sheets that can be publicly accessed by a link provided. then you can freely share your sheet with the GM before even having access to their game and discuss ideas, then its just a case of transferring the info into roll20 once your able too. hope this helps, welcome to roll20, best of luck finding a game!