This page offers general tips on picking a good topic: http://www.sciencebuddies.org/mentoring ... tion.shtml
In your case, there are many ways you could go about finding a topic on recycling. For example, you could look at which material is the easiest to recycle, or how much does it cost to recycle the different types of materials. You could also investigate the different recycling processes and techniques further to come up with your assessment on what is the most efficient (from a time/cost/environmental hazard perspective) process. Recycling is a big thing for the future so there's a lot of interest in this field right now. Try to do a Google search after you figure out which specific area about recycling you want to go into. Alternatively, you could look at conservation (for example energy), which is another hot area to investigate that's along the same lines.