As Futtawuh says, it would take a very long time to explain how to do this, because there are so many other factors. Some suggested starting points would be:
1. Power search (Top right hand corner) is your friend. There is alot of helpful information scattered about the forum. Most questions will have been asked before. You should be able to find what you need.
2. You will need to get root privelages for certain things. Look for rootsh, using ssh or terminal. Filebox is also very useful.
3. This thread is all about modification to the n900.http://talk.maemo.org/showthread.php?t=39565 It is a long read I know, but there is a lot of useful information in it. Take time to read it.
4. This all might seem overwhelming to start with, but really, it is faily easy. If you are interested in changing the look of your phone, then it is essential to learn about these things. This is just to give you a couple of ideas of where to start. Hope this helps. The best thing is to just have a go. You will be surprised at what you can do with this phone.