If you don't have color on your hair right now, then take your desired hair shade level #, multiply if by 2 and minus your existing hair shade. For example, you may want level 10 and you are an existing 9. That equation would be D X 2 -N, or 10 x 2 - 9 = level to use. Therefore, you will need to use level 11. Match your existing color to a swatch at a beauty supply store, look for your desired color level, do the equation. Also, you will need to look at the tone of your existing hair color, do you have any red or orange in your hair? If so, you will need to use your desired level with ash or blue to not lift and have a brassy look. To get rid of a non-desired shade, go directly across from that shade on the color wheel. Therefore, if you have orange, you will need to have some blue mixed with the color to get rid of it. You will need to get 20 or 30 volume developer. The products that you need can only be bought at a beauty supply store. Don't buy a "kit" at the grocery store.
Color can't lift (make lighter) color. What that means is, if you have color on your hair already, you can't make it lighter using permanent hair color. You will have to use bleach to lift the existing color. Make sure you use bleach that can touch your scalp. Many times when this is done by someone that has never done it, they will get chemical burns on their scalp. Be careful.
My suggestion, is to have it done by a professional. If that is out of the question, go to a school and ask for someone that is about to graduate. They have to go to school for 1500 hours. This means they go all day, every day for almost a year. If you go to a school, you will most likely have to sign a waiver of liability. AKA, if your hair falls out in the sink, they aren't liable. Good luck.