First of all you have to find the cause of the appearance of patches on your skin. They could be due to allergy or some other skin infections or can be also due to adverse reactions of some medicines. It would be better if you consult a good skin specialist. Please do not delay or neglect this as it could mean some serious issue.
For dark patches, you can try home remedies. Home remedies are safe, effective and also cost nothing. You can try sandalwood powder and turmeric powder paste. Mix the two [powders together and make a paste with milk or lemon juice. Apply on the dark patches and leave fro about 20-30 min. You will see the patches fading away but this being home remedy takes a lot of time. You have to continue for at least a month to see some result.