There is nothing more annoying than a dripping tap. Here are some tips on how to repair a dripping tap.
First of all is to identify what type of tap it is. There are three main TYPES:

1. Tap washer (the traditional tap ,will normally have tap heads to switch on and of)a separate hot and cold and the tap heads will turn approximately three complete turns from fully on to fully of. There are lots of sizes of tap washers so you can buy a selection box of washers but this would have around a hundred or so washers in it or take apart tap and then go of looking for the correct size of washer. Also tap washer tap insides can be prone to locking up when they get old and a pair of new insides can be bought(they are commonly called tap revivers which most d.i.y shops and all plumbing shops will sell)however older taps can be different sizes with different threads to the new taps which are now normally one size.
2. Ceramic discs ,will have a separate hot and cold ,will normally have levers and will do a nighty degree turn from fully on to fully of. With this type approximately half of the taps take one common part to fix them (the whole tap inside part needs changed no repair can be done to a faulty one) and the other half need unique parts made by the tap maker (you have to identify the exact make and model and the parts still available).
3. A SINGLE LEVER WHICH GOES UP AND DOWN AND LEFT AND RIGHT: This type the parts inside have to be replaced with new and the part has to come from the maker (the exact make and model has to be identified and the parts still available) it is quite common with this type to just replace the tap with a new tap.

I hope this information has been of some use and if you are not sure or do not feel confident then your local Aberdeen plumber with twenty five years experience would more than happy to fix your tap for you (no job to small).