Well, for starters, I'd take care of my inferiority complex and my severe lack of self-confidence. I have low self-esteem and I generally belittle myself (ironically, I'm still happy with my life). I would also try to improve myself in social situations. I have somewhat of a social phobia, which makes it hard for me to communicate with people I don't know. Although, if they start the conversation, then I'm fine. I just can't be the one to speak first. I also have a terrible fear of speaking in front of others.
All of these things pretty much stem from my lack of confidence, though, so I think if I can find a way to get that under control, I should be fine with the other stuff in the long run. My inferiority complex and my self-confidence (or lack thereof) are definitely the biggest things I need to improve about myself.