Be realistic.
Some people don't believe in themselves because they set impossibly high expectations for themselves. So don't get down on yourself when you don't live up to something that few people can do. For example, let's say you would like to start running. Don't sign yourself up for a marathon a month later and then be disappointed when you don't finish-people train for them for months! Cut yourself some slack and be realistic about what you expect to achieve.

Practice positive affirmations
Your mind is a powerful thing. That's why being positive about yourself and your abilities will help you to believe in yourself. Throw out any negative self-talk (I can't do this, I don't even know why I try, That person is better than me, and so forth) and replace it with positive things. This will take practice, but soon you will find that instead of negative thoughts, the first thing that comes into your head is a positive one. Try this: every time you have a negative thought about yourself, immediately replace it with something positive. For example, let's say you want to take a kickboxing class but are embarrassed because you think you're too out of shape. Instead of entertaining that idea, say, "This will be a fun way for me to get in shape and meet other people who have the same goals as me." If you tell yourself you can do it, you can.

Surround yourself with positive people.
The attitudes of those you surround yourself with have a great effect on you as well. That's why it's so important to associate with people who are encouraging and positive instead of pessimistic or who downplay your abilities and your dreams. Never let anyone have power over you to tell you what you can and can't do. You know yourself better than anyone-if you think you can do something, go after it and get it and don't let anyone else bring you down.

Just try.
Most people who don't believe in themselves are in the habit of letting opportunities pass them by or not trying things because they think they're going to fail. Instead of not doing things or passing opportunities, make a promise to yourself that you will try new things no matter what. You might find that you're smarter, stronger, and more confident than you gave yourself credit for.

Learn from mistakes.
You're not going to succeed at everything you do, but that doesn't mean you shouldn't try or that you should get discouraged. Instead of getting down on yourself, learn from your mistakes. Sit back and assess the situation and decide what you would do differently the next time around.

Believing in yourself is a key part of having a happy, fulfilling life. If you need help believing in yourself, the above tips are a few ways you can have more faith in yourself and your abilities.