Rugger hit it spot on. The key to looking good is fit, coordination, and non outdated clothes. Some advise on how to look your best on a limited budget based on personal experiences.
Fit: Do not not make the mistake that most people make and wear clothes that are too big, simply because they feel more comfortable. You also do not want clothes that are skin tight on you. Find something that is in between.
Coordination: I am in the same boat that you are, I have a low income as well. My appearance improved drastically once I learned about how to coordinate colours. What colour pants, shirt, sweater, jacket, shoes look good together. It is not just the colours but also shades of colour. For instance when will a light blue shirt look good, when a dark blue shirt will be too much. You could wear the most expensive shirt and jacket but if the colours do not match, it will look awful. So take the time and do some research on how to best combine colours.
The last point is wearing outdated clothes. Thrift stores are great places to find bargains. But what you make think of as a bargain may well not be. I thrifted an Armani jacket on ebay for 10 dollars. The jacket is in excellent condition, except for one problem. It is horribly outdated. (Which is the reason why I won the jacket at such a low price) The shoulders are two inches larger then what I normally wear (what some people call linebacker shoulders), the lapels are large and sit very low. It is also very baggy. No matter how much I have tried to make it work, it does not.