depends on what adhesive you use. If you are using an acrylic adhesive then i wouldn't try to bed them out more than a couple of millimeters but by the sound of it your walls are pretty uneven so a cement based adhesive sounds more appropriate. The cement based ones are generally much better (they are all we ever use!) than the ready mixed but also have there limitations. Assuming you go for a cement based one it will go straight onto plasterboard or render,plaster will need priming, existing tiles will need priming with a specialist primer, timber (plywood) i would avoid unless you use a flexible one. Some cement based adhesives are great and some are hopeless. A good one which we never get any trouble with is keraflex and if you need to bed out a lot i:e >10mm than keraflex maxi will go even deeper both made by a firm called mapei. Anyway whatever you go with try to start your tiling in the middle of the wall and check with a straight edge along the face of the tile that the tiles at either end will not have to much or to little bed,similarly check that as you go up the wall you are not going to be putting to much adhesive on or be struggling to get them in . When you 'dob' the adhesive don't just put 4 blobs on the corners but try to get about 75% coverage once the tile is pushed back normally a 12"x12" tile would have about 16-25 dob's on it ,the general rule is the thicker the bed the fewer the dob's otherwise the tiles start to behave like a big wobbly jelly