hi 
Skin tanning in most cases resolves of is own....all that you need to do is to apply a sunscreen(spf>26) regularly over your face...you should prevent any further damage by sun by using a sunscreen everytime you move out in the sun.....Just wait for 2-3 weeks and if even after continued sunscreen use, you feel that the skin colour is not coming back to normal,,then you can use a lightening agent like 
hydroquinone once a day...but remember that such agents are used only during night time..... you can also take oral 
vitamin c supplementation along for a month
If inspite of all these measures, you feel that the pigmentation continues to persisit, then i would advise you to see a 
dermatologist who might be able to help you with other techniques like 
chemical peels, 
dermabrasion , lasers etc 
Now as far as the rashes of 
prickly heat are concerned, you can use 
calamine lotion over the rashes in order to decrease the irritation..In order to avoid such rashes, wear only cotton clothes ,so as to decrease sweating,,Do not apply any oils or moisturizing lotions over your body as such things tend to obstruct the sweat outflow, and thereby increase the development of miliria lesions
Hope it helps
Dr Geetika paul