Don't worry about it too much!
Practice is the key. The biggest thing you may get wrong is applying too much paint and creating runs, but its not like using spray/rattle cans there isn't a lot of paint comes out of an airbrush (unless you really go for it) and if the brush is a dual action type you can control that anyway.
I'm new to using an airbrush, although I have spray painted motorbikes (real size ones!), and my first attempt came out OK after lots of web surfing and a bit of practice with ink as Tony suggested.
My first go is here:
http://www.scale-models.co.uk/under-...b-mustang.html (Revell P51B Mustang)
Now thinning the paint is the tricky bit, I'm useing enamels at the moment and haven't quite got the hang of it yet! They all say thin until it's like milk, but I can't quite get the right combination of thinner and pressure yet.
Good luck but most importantly have fun!