I tend to use the non drip ones, the ones that take 10 mins. Top tips have lots of old towels handy if it is your first time, then cover your bathroom in the old towels, and wear old clothes, in case of drips make sure you have a watch, I normally sit and read a book while waiting have some cotton wool and some hairspray and gently wipe around your hairline and the back of your neck and the tops of your ears to remove the stains
Start with a temporary in case you well and truly bugger it up. When I was 15, I did one of those semi-permanent colours (Inecto Hint of a Tint) in copper - ended up with a bright magenta patch on the back of my head where I had first slapped on the contents of the packet from my hand - 90% of the colour "took" instantly in that one area, and the rest was perfectly normal.
So - don't slap it all onto your head and then spread it around, spread it around your hands first and work it into your hair as quickly as possible.
Dont' change colour too drastically, especially if you have sun-bleached or grey hairs as they will absorb the colour MUCH faster than normal hair.
Wear plastic gloves as your dry skin and nails will take up the hair colour as well.
vaseline around the hair line and ears helps avoid "tidemarks".
thumbwitch, I had a similar experience with a 'shader and toner' at 17: my fringe was richchestnut orange, the rest was dark blonde. I washed thebugger my hair for ages, ooh - at least 40 times that day, it wouldn't shift. Not a good look.
I advise going within about 2-3 shades of your natural colour. That way if it is a total disaster it won't be too tricky to get a hairdresser to sort it out (without wrecking you hair). If you are light naturally, don't try to go too dark. (a) it's a shock and (b) it can take unevenly. Good luck.
my top tip, is colour wise, if you just want to enhance your natural hair colour/cover the grey, go for one that has no ammonia in it. most hair dyes work by stripping out the natural colour, then replacing it with dye, the ammonia is the bleach.
ammonia free dyes, just colour, so there is less fading over time. one with ammonia in it, will fade quicker, which means your hair will gradually go lighter as the bleaching is revealed.
also, home colours tend to go redder as they fade. the red is the strongest pigment, so unless you really want red tint, stick with cool/ash colours.