Your free player is essentially a permanent 'slot'. If you create a free player and decide you don't like him and retire, you'll have the ability to create another free player. It never goes away or anything like that.
Create now! You'll be ok (can still hit the max level (79)) even though the super optimal is creation during the offseason. It's only a little missed training time. I would suggest making some sort of defensive player (they tend to be my favorite), specifically a LB or CB. These positions are always in decent need by teams and they get a good quantity of plays so you have plenty of highlights to watch.
Oh and make sure you do things moderately 'right'. GLB is a big learning experience where your generations of players continually get better and better however starting off a good foot is a big help.
jdbolick gave a little simple (direct) advice on how to train a dot initially:
http://goallineblitz.com/game/forum_thread.pl?thread_id=4959435&page=1#45992357I'd also recommend reading Rage Kinard's old guide:
http://goallineblitz.com/game/forum_thread.pl?thread_id=3266213 . it's a little out of date now BUT it does a decent job at explaining some of the simple principles behind efficient training and SP application.