If you don't have tiles covering all your bathroom's walls, it makes sense to put in a tiled splashback above your bathroom sink, as this area is prone to water splashes.  The key to doing a job here is to measure properly to ensure you position your tiles centrally.  You'll need to mark the central point along the back edge of the sink and use a spirit level to draw a vertical line with a pencil here.  Take your first tile and mark the middle of it along its top edge, to help you line it up centrally with your pencil line. 

Apply adhesive to the tile using a notched spreader to make grooves.  Put it carefully into position, lining up the mark on the top with the vertical line.  Work from this central tile outwards, using spacers if you like, to help ensure the gaps between the tiles are the same size.  With the first row of tiles, it's a good idea to slot small pieces of cardboard under each tile to make sure they are all level.  The same applies to the bottom row if you're using border tiles.  If possible, use whole tiles for your splashback, as if you have to cut tiles they will be clearly visible.  Don't worry if the tiles overhang the sides of the sink slightly.

Adding border tiles to the corners of the splashback is a nice finishing touch.  If you need to use an electric tile cutter, wear goggles to protect your eyes and follow the user instructions.  Once all your tiles are up, give the adhesive adequate time to dry and remove all the spacers before grouting.  If you find it difficult to use a spreader on tiles with a raised surface, wear protective gloves and use your fingers to achieve a better finish.  Once the grout is dry and you've cleaned off any surplus bits, fill in the gap between your tiles and the top of the sink with silicone sealant.