Do invest in a test kit, even a cheap one if you really can't spend much money. We need to know your pH, GH, NO2, NO3 and ammonia readings, plus KH for CO2 readings. What size is your tank? if suitable some snails eat algae (getting the wrong type means they'll eat your plants too, ask another member for advice for what type of snails, but aqua essentials sell them, so I'd buy from there or a reliable LFS.)
Bristlnose plecs eat algae, as to otocinclus for the small aquarium.
Do you use a magnetic algae scraper? many people don't like these and they can scratch your glass, but when used with extreme care they can get the worst of the alage off.
You need to find the underlying cause of the algae, and to do this you need a comprehensive test result, and we need information like your tank size, it's occupants, and whatever else you can tell us about it, espcially your maintenance regime and it's hardware.