The best way to start practicing is to put out a still life and draw it with your watercolors. For that particular medium, I really like using flowers, fruits, or anything really colorful.
One thing I can say about watercolors, though, is that if you're starting off, try to use a thicker paper. A good paper can withstand pretty much anything you throw at it. The Moleskine Watercolor sketchbook is also pretty ideal, it's got the thickest paper I've ever used (it can withstand strain from both sides of the paper!).
Also, here are a few tutorials I found immensely helpful.
EcthelianGold-Seven - Part 2. There's also 1 and 3, all good.
Maevachan