Luck is the success or failure that is apparently brought by chance rather than the actions of somebody. In a sense, luck does exist. Believing that you're going to be lucky is basically just a pick me up, you will find that many athletes or gamblers, in fact some of the best, have certain superstitions or rituals that they perform before their next game or gamble. For instance, I shall use football (soccer), one of the best football players, Ronaldo, has a superstition in the changing room that he must change his hairstyle at the half time mark.
Really, you have to believe in either yourself (your performances and actions), luck, or fate. If you don't believe that you yourself brought this success or failure upon yourself, such as not studying for an exam, thus you fail, or doing the opposite, you must believe that you had either good or back luck, depending on the outcome, or something that is believed by religious people widely that their God (whatever higher being) chooses their path (known as fate) when they are born into the world that they created.
For me, I do not believe in luck. Luck is simply a pick me up or put me down, it's emotional. If you think you're going to luck out in an exam, you are probably confident, and if you don't think you're going to be lucky, you will state that to your friends afterwards (likely), which indicates that you're not confident.