I have asked a few questions on Stackoverflow, and I am grateful for the help I received. I know that I will continue to learn by asking questions here. I am most familiar with HTML and CSS and, though I am far from an advanced user, I am sure I could answer some basic questions. However, when I search for questions almost everything I find is far too advanced for me. Is there a way to sort questions by complexity? Are there any sort of beginner/ intermediate/ advanced tags?
Don't feel like you should be obligated to be searching for questions to answer. If you see a question you can answer, then answer it! If not, then maybe look for answers and research. If you can't figure it out, wait for someone to answer, and learn from it. That way if new questions that use the same technologies/concept pop-up, you will be able to answer them :)
Like I said, you shouldn't feel like you have to search for questions to answer -- StackExchange rewards people for not only having great answers, but also just as equally people who have great questions. And may I add this was a great question :)
Complexity...is difficult to automatically determine. So I don't think that anything automated could come along to sort a question by a given complexity.
My advice to you (if you don't want to read the wall of text below) is to answer the questions you feel like you can, and if you're not sure, do a bit of research too, and see if you can come up with a solution as well. You'd be surprised about how much better you'll get if you continue to answer questions.
Now, if you do want to read the wall of text, I've got a small spiel for you.
When I first joined SO, I was still a junior/senior in college; my experience with Java (and Python, at the time) was basic, but I still felt that it was strong enough to answer questions.
So, I started answering questions. I answered every question I felt that I could.
I got some very correct. I got some very wrong. I gained rep, lost rep, got accepts, got unaccepts, deleted some answers, and so forth.
Now, I'm further along in my career. I feel much more confident in my skills than before, and I attribute it to answering a lot of the questions. In that, I discovered so much more about programming, and about Java in particular - stuff that was out of reach a couple of years ago.