Tiling a room can be quite a daunting prospect and it's quite easy to make a complete mess of it so take your time and work carefully and methodically.
The key is preparation and planning. Make sure your surfaces are suitable for tiling onto(see preparing surfaces). Plan the layout of your tiles so you know where they will all go before you even think about laying the first one.
And remember, if you are tiling a whole room you don't have to attempt it all at once. Doing it in sections is fine as long as you remove any surplus tile adhesive before it dries