The first clock you work on should ideally be one of simple design and relatively low value.
Avoid touching anything of sentimental value as you may end up having to pay for the repair, and try to find one that does not strike. If you must choose a striking clock, leave ones that strike on the quarters alone. Wait until you have assembled an ordinary striking clock and understand their workings thoroughly.
If you want to overhaul a clock of sentimental value, practice on an old one first. These are plentiful in second hand shops, but watch out for broken mainsprings as the impact of breaking can damage teeth on wheels. Check that the clock will wind up fully, and if going to an auction it pays to take a bunch of assorted winding keys or a star key.
Pendulum clocks are easier for the amateur to work on than hairspring models. Check that the movement is complete. Old American and German clocks are good to start on, whilst French ones are very delicate. English clocks are generally too expensive to practice on, except for the mass produced models around in large quantities. Many of these are in 'Napoleon Hat' style cases.
Repairing Your Own Clocks by Mervyn Passmore