First things first: how long have you had your tank? What fish and how many do you have? How big is your tank. What are your water parameters (ammonia, nitrite, nitrate, pH) and most importantly: is it cycled? (if you don't know the answer to this question please follow the link in my sig and read my article).
Brown algae is usually the result of two things:
1. Insufficient light
2. Excess nutrients (either ammonia if the tank is not cycled, or nitrate and phosphate if it is).
These nutrients can occur as a result of over-feeding your fish, not doing sufficient water changes or (annoyingly) because your tank water is full of nitrates and phosphates. Over-stocking a tank can also be a causative factor.
I'll wait until I get the answers to my questions before advising you on the next course of action.