I have small pimples since december what is the best thing to do and I have dark spot too get this when I have pimples?

Doctor can evaluate. Rashes are hard to describe in words. Small pimples can be due to acne reasons, which can happen even with adults. The treatment would be acne medicines, which include benzoyl peroxide, retinoid gels, and antibiotic gels. Small pimples can also be folliculitis or other skin problems. Darkening of the skin after any rash can happen because more blood flow and more pigment can occur during healing.
Exclude melanaoma. Firat you should see a dermatologist for a tentative diagnosis and to exclude the possibility of a melanoma which should be biopsied if suspicious. Topical antibiotic creams or retinoids or anti-androgenic meds may then be considered.
Several Options. Physician grade skin care products will cleanse your skin and get acne under control. Microdermabrasion and chemical peels can significantly improve acne scarring, and dermarolling is most excellent. The pigmentation can be treated with intense pulsed light (ipl, or fotofacial) or a lightening cream.