“You experience what you believe, unless you believe you won’t, in which case you don’t, which means you did.” This is from Harry Palmer of Avatar, who has an "interesting" history. Be that as it may, this is an excellent quote on positive thinking.
The underlying idea is that your beliefs create your reality. Now we have to look at 'reality' a little bit here... To borrow from ACIM, I'm going to use reality as the world we see around us, everything separate, and our eyes trusted; and Reality as a deeper truth of metaphysics - everything interrelated and requiring the perception of our intuition and feelings.
So, at the level of reality, your beliefs affect you, not the world. Even here, your beliefs change your perception of and your reaction to the world. Therefore work on your beliefs changes the world for you. Consider an example - 'I can't pay my bills'. If this is your belief, you miss possible opportunities to get out of debt. If someone presents you with an opportunity, the usual reaction is, "I can't pay my bills, how could I afford that!" Thus it has become self-fulfilling by closing your eyes to any other possibility.
At the level of Reality, we are a part of everything around us. If that is true, then we have a strong influence on the world, similar to the influence you have on getting your sister to turn on the lights for you. You ask, and she (probably) is happy to help out. Thus 'I can't pay my bills' attracts more bills to you; it tells money to stay away as this is what you're calling to. As long as you hold that belief, the world agrees. And as soon as you change that belief, the world agrees.
Which means whatever level you live at, beliefs control your reality. And you control your beliefs! By changing your beliefs, you can change your reality. Now, how do change your beliefs. Well, first you have to be aware of them. You can't change what you don't know about. Once you've found a belief, then you have to decide if it's helpful or harmful, and then decide if you want to lose it. Luckily, by looking at how you feel about your belief, you can see whether you want it or not. Notice how 'I can't pay my bills' makes you feel. How about 'everybody in this town drives crazy!' Or try 'this is a beautiful day, and I'm lucky to be here.'
As the adage goes, nature abhors a vacuum. It's nearly impossible, if not impossible, to drop a belief. It's much easier to overwrite one. That's why Affirmations are so popular. You (or a coach, partner, etc) find some belief that isn't working, turn it around into a positive affirmation, and slowly overwrite your initial belief. You can't believe that you are smart and stupid (for example) at the same time, so one will win out. Which one is up to you.
Your beliefs were given to you by trusted sources: parents, friends, teachers, life experiences. Can you be your own trusted source? Maybe, but it's harder. If your teachers told you that you were stupid, you're going to have to work hard to convince yourself otherwise. But you can. Do things you know you can do; challenge yourself a bit and see that you can do things you thought you couldn't; daily tell yourself how smart you are; have an experience proving you are smart; ask your friends to let you know when they see you do something smart. It'll take a while, and we're good at undermining our own efforts, but with work you'll do it.
Then you'll start to see the world differently - through 'smart' eyes. And you'll find all kinds of opportunities to show everyone how smart you are where you used to only see how stupid you could be. As Paulo Coelho says in his great book The Alchemist, the world wants you to succeed.