Daily and regular sun protection is a must. Apply a sunscreen with SPF of at least 50 at least twice daily. Just by doing this religiously, you may be able to arrest further worsening of the condition.
Do consider seeing a dermatologist who can evaluate the depth of your melasma and determine its prognosis (melasma that is "deeper" will be harder to treat). Medical treatment with lightening creams and creams that normalise the cellular differentiation of the upper skin is usually the first-line management recommended. Other treatments which can be added later include exfoliation either with creams or professional chemical peels, certain oral medicine, laser or light treatments can be discussed too.
Above all, be patient – there is no miracle cure for melasma.And be warned – melasma may recur after any form of effective treatment; so always be mindful of sun protection.