Don't envy you - I've done this in the past and it's a nightmare trying to rectify it.
If at all possible, I'd recommend going to a hairdresser to rectify this, especially if you're inexperienced with using hair colour.
However, if you're determined to do it yourself, buy 1 or 2 boxes of 'Loreal Perfect Blonde Pre-Lightener' (depending on how long your hair is). This product basically strips ALL colour from your hair. For the love of God, do a strand & sensitivity test before applying it & time it very carefully. I can't emphasise how harsh this product is - pure peroxide, so take care & make sure you cover all clothing and surfaces. Once you've done that, you can apply another colour on top. Your hair will feel and look like straw, so you'll need to use intensive conditioner for the first few times when you wash your hair afterwards. I would really recommend you go to a professional for this process though...