For best results you need to be tiling on a sound even surface.
If you try to tile on an uneven surface you will quickly find that the tiles don't line up properly with each other and the end result will look "lumpy" and quite amateurish. I have tiled on uneven surfaces in the past but find I had to end up putting extra adhesive in certain places to level up each tile up as I went along removing tiles that looked bad and generally spending far far longer on the job than otherwise.

Tiling on walls
So if your walls are bad you may need to consider having them re-skimmed(with plaster) or re- rendered(with cement) by a plasterer.
Alternatively put up new plasterboard.

If your tiling in a bathroom there is a great product which is now available to DIYers. Tile backer board is a tough water proof board. It' a bit like hard plaster board, but made from a rubberised compound. It's lightweight and tough but also water proof. It specifically designed to be used in wet areas like bathrooms. Its made by WEDI . Checkout their web site. They've got a very useful how to guide and a list of national stockists. Check it out at http://www.wedi.co.uk/flash.htm

Tiling floors floors is fairly straightforward however it is particularly important thing to ensure is that you have a solid flat surface otherwise after a while the tiles will start to crack. Usually the bigger the tile the more likely it will be to crack on an uneven surface.

How to tile an uneven floor
Concrete floors are ideal to tile onto. If it isn't level you can buy a self leveling compound which you mix from a powder and pour on. (it looks like runny cement). You spread it out a little with a cement float then it finds it own level filling any hollows.

It's not advisable to tile directly onto floorboards, you'll need to lay a minimum of 12mm thick sheet of ply wood first. Screw it down securely with lots of screws

Its not essential but II also like to prepare floor surfaces by painting on PVA sealant(readily available at most DIY shops) which helps adhesion and waterproofs the surface.