You can't get the practice without trying. As others have suggested, I'd pour a small slab first that won't be the end of the world if it gets screwed up. My wife and I invited three friends over and thought that we could pour our basement slab ourselves. What a mess that turned out to be - one Maxxon overlayment professionally applied later, we now have a good finish. For less money, I could have had a professional crew come out and it would have been perfect. Now I have a gypsum-based skimcoat over my nice hard concrete. Sigh.
AFTER this pour, I poured a slab for my 300 gallon diesel tank. That taught me the amount of work it takes to get a truly nice slab. AFTER that, my wife and I poured a 12x16 slab for a small outbuilding. Both of these little slabs turned out great.
What I learned from each:
1. Site prep is king.
2. I sure am glad that I own all of the right tools.
3. Site prep is king.
4. Rebar on a closer schedule than what is truly required is not a bad thing.
5. Factoring in short-haul fees, etc., redi-mix is the way to go - portable mixers are just a pain and decrease the amount of working time you have with your slab.
6. Pour on a cool, almost cold day, first thing in the morning if you are new at this - the extra time you'll have with the slab no price can measure.
7. Pouring when it might freeze in the coming days/nights is to be avoided.
8. Laying down some plastic under the grade is a good idea and cheap.
9. Site prep is king.
10. Cover the slab with plastic sheeting to assist the cure.
12. Broomfinish covers lots and lots of mistakes.
13. There is limited learning from watching. Playing a part in a pour is the best teacher.
14. It's always good to have slightly too much concrete ready to go than just slightly not enough.
15. Understanding the above, if you have some other little project formed up and ready to go, you can use that form for any overages.
16. Site prep is king.
I'd grab a how-to concrete book and try something small that doesn't *really* matter before I made any decision to do something that I really cared about (like a patio).
Good luck!