Although I'd personally recommend Silo, the best thing I can tell you is find one modeller and simply stick with it. No program out there is going to take a couple of days to master. Modelling is one of those things that you never seem to master, but rather just get better at it. And unless you are a complete wiz, you won't be making anything you'll want others to see in just a few days.

So find one (the suggestions so far are good choices) and just go with it. Use it all the time, even if what you make is stupid. Find every tutorial for that program and do them, even if it's for things you'd never normally make. See if there are any tutorial DVD's or anything like that you can buy and use those as well. If you don't dedicate a good portion of your time to learning your program you'll only ever be a so-so modeller.

There's not a day goes by that I don't open up Silo and make something in it. 90% of the things I make never even see the light of day, but it all goes towards helping me understand the program better and stretch my skills as a modeller :)