Faith is a huge topic. Many people spend their entire lives studying their faith, usually it is their religious beliefs, but often it is faith in other things that are not religious. Most people do not dedicate their entire lives to their faith, but the dedicate part of every single week to their faith by going to their place of worship (like churches or temples or mosques). One letter from me will not clear up all the questions that you will ever have about faith, but what I hope is that this letter will help you understand that faith is made up of four parts: believing, thinking, feeling, and choosing. All four of those things go into faith.
What is faith? My favorite dictionary definition is “confidence or trust in a person or thing.” You have faith in someone when you believe that they are good for you. And you faith in an idea or a thing when you believe that idea or thing is good for you. If you believe that something exists but it is harmful, then you may believe in it, but you do not have faith in it.
My favorite non-dictionary explanation of faith comes from the Bible. Hebrews 11 says a lot of things about faith, but verse 6 is my favorite part: it says that having faith in God means that you believe that God is, and that seeking God will be rewarded by God. I think that is a a great definition of faith in God, of course, but I think it applies to faith in other things as well.