Practice! Check out tutorials, there's lots of them on mut when you have enough posts to see them. Look at pictures in magazines and try to copy the looks you see, etc. You can't learn overnight, it takes some practice! And you can always ask us questions.
Oh I also wanted to add that a lot of what can be the difference between amateur and professional looking makeup is blending. Good blending technique and blushes make the look more natural and can allow you to pull off colors you didn't necessarily think you could. In my experience, anywhoo.
find out your mistakes and try to fix them. do your makeup and ask someone who knows a thing or two what you did wrong so they can tell you how you could fix it. sometimes we think we did our makeup good but dont realise theres something wrong with it so take pictures and compare them to proffessional makeup pictures. learn all you can on the internet about tecniques how to do certain things like blending, what colours to use in certain situations/ lighting how it affects your makeup buy books to help you and keep in mind you will not be able to do professional makeup unless you become one. but you can be very good at it.
oh and one last thing makeup needs talent. some people are just bad at it.
Practive, experiment, just sit in your bathroom and have fun with it. That's the best thing you can do. Also, ask the MA's lots of questions... they will help you. Websites like this one and Specktra.net are a great help. Makeup is meant to be fun, so enjoy it, and as you experiment you will get better.
Before I joined MuT, I'd go to makeup counters at departmental stores and if the MAs were not busy, they would do my face for me and teach me tips and tricks. I learnt a LOT from them. I learnt a TON from Mut, esp. the Tutorials. And like the rest have said, you see or learn something you like, go home and try it.. practise..