Serious Eats - seriouseats.com

First thing is avoid the Ramblas area for eating - the food is overpriced and a long way from being the best in Barcelona with the exception of la Boqueria market:
http://www.barcelonatravelogue.com/ramblas-barcelona/

Instead head to El Born, although it can be hit and miss, but I'd definitely recommend Senyor Parellada as the food is authentic Catalan and very cheap for what you get.
http://www.barcelonatravelogue.com/barcelona-restaurants/review-senyor-parellada/

My favoite restaurant in Barcelona is Restaurante Carmen near Sants station. It's unpretentious, untouristy and specializes in grilled meats, but may be too pricy for you - it's not expensive, but not cheap cheap :)

If you like Indian food then check out El Gallo Kiriko (C/ Avinyó 19), which is really cheap and right on the old city wall - it's a popular place for students, so might suit you well.

Finally, most restaurants have a Menu del Dia, which offers a limited menu during week days. Usually they are around 8-10 euros, but I saw one the other day for just 5 euros including wine, though I can't say how good the food is for that price.

Avoid tapas - I love them, but they add up if you want to fill up. But if you do want to go for tapas then head for some of the Basque tapas bars in El Born which are really good - I love then one opposite Estacio de Franca, but can't remember it's name at the moment. Again, full of locals, so very gebuine.