In my experience, padlocks are the easiest to pick, followed by deadbolts and then door knobs. I bought a set of lock picks many years ago and tried them out with little success. Then after getting fleeced by a "locksmith" who wasn't even able to pick my front door locks (he had to drill out the cylinder), I decided to give it another more determined effort. Now I can pick pretty much any lock ANSI grade 2 or below. Grade 1's are a bitch and I've only been able to pick them under ideal conditions (cylinder removed, in the privacy of my room under optimum lighting).
If you want to practice, I would get a cheap grade 3 lock and remove the cylinder and just practice on that until you get the hang of it. Then step up to grades accordingly after you've mastered them.