There is no simple answer to this question.
If you truly believe in magic, the odds of a spell working will be significantly higher than if you are doubtful, or don't believe at all. This is because most of the energy used to power a spell comes from within, and faith plays a huge hand in whether or not it will be successful.
Another factor that plays a part in this is whether or not the spell is something that could really happen. For example, a spell meant to help you find a new friend will be much more effective than, say, a spell meant to make someone grow a tail. Be realistic.
I could go on all day arguing whether spells actually work or not, but ultimately, it's up to you to decide for yourself. There is a lot of controversy surrounding the subject, and it's more of a personal venture than a public one. Believe what you think is right.