Accept and admit that you need to change. This is a huge step. The fact that you already clicked on this article shows that you want to change, and know somewhere you have a problem.

Think of all the things in your life you want better. But make sure that they're positive. Maybe you want to change your habit of always judging people? Or maybe your habit of being negative? Make a list of all the things, or the thing, that you want to change.

Don't set any limits to your change. If you think maybe, someone will make fun of you for trying to change, or maybe it isn't possible, or that you just can't, realize that that is a dumb, negative thought! It is very possible, this is all just in your head, all those negative thoughts, and those thoughts need to change also.

Write the steps that you need to take to achieve the changes you wrote down. Be reasonable, but don't be afraid to reach high at the same time. Set a day when you'll start taking all these steps.

When that day comes to start doing those steps, just do it. Don't even think about it. You are always doing something, you are always active. Even when you're watching TV, or going online.

The thing is, to actually be proactive instead of negative active.
Make a habit of these new steps. It takes 21 days to break a habit. The first day will be so hard, but then it gets twice as easy every time.

Tips
Remember, that you always should do the right thing. There is a such thing as good and evil, and you need to always be on the positive side. SO if someone is worried about you because you're being negative, they may be right, but if someone is bringing you down, don't pay attention to their negativity.

Forget about other people and their comments, your life is yours.

Don't let people tell you how to live your life. Be yourself, and be true. Stay true to yourself, and others will be true with you. Don't ever doubt yourself...

Travel somewhere to clear your mind, maybe you'll discover new things or new thoughts that may change your life!

Read some philosophical thoughts. Maybe you'll find that your personality existed many centuries ago.

Changing your appearance is the last thing that you should work on, because what really changes anyone is his/her inside not outside.