Yay - go for it! Jocelynif nailed it with the "process"-comment. Try riding other people's bikes and try to note down the differences - what do you like, what do you not like. Riding in an upright position will slow you down but let you watch the scenery more easily, riding too stretched out will give you neck and shoulder pain but can let you go fast. If the saddles you try are awful you can always spring for a good saddle already now, and transfer it onto your bike once you buy one. And remember that the bike you buy doesn't have to be perfect, you can do a lot of tweaking on a bike as long as it basically fits your needs. As long as you're comfortable and the bike is lubed, has good brakes and plenty of air - you're gonna have fun.