I assume you are still very young?
If this is the case, you have more than enough time to begin correcting imbalances. The good thing is that you have identified the problem areas. However, like gone heavy said you need more muscle than what you have acquired. That being said:
It is not uncommon for one side to be bigger than the other. When you say muscle imbalance do you mean size related or are you having complications due to imbalances such as shoulder pain or knee pain?
I submitted a post about imbalances a while back. I discussed how young people should research before they begin lifting. The number one routine for new guys-BENCH-CURL_BENCH-CURL-BENCH-CURL. This leads to shoulders rounding inward causing pain in the shoulders and back.
If it size issues, just add a few extra reps and use DB for the exercises.
You will progress past this stage, so dont give up. You have identified the issue, know do what it takes to overcome. Good luck!