Kovu... the rule when dying your own hair is not to stray further than 2 shades from your natural colour. and follow the instructions on the packet!
if you want to go more than 2 shades darker, use a colour that washes out in six washes, just in case of disaster (if you have to mix the colour, i.e. there are two bottles in the packet, this is not a six wash colour). Then if you are happy with the shade, you can then swap to a semi-permanent in the same make and shade next time.
never use a permanent colour. semi-permanents (24 washes) are what you need as they become permanent over time, as you layer the colour on. when re-applying always do your roots first (leave colour for 10 mins before applying to rest of the hair)
or... as suggested... contact the local college. they do it for about a fiver. Or try a "training" night at your hairdressers.