Pensar - To think
Creer - To believe
For example:
Estoy enamorada. Estoy pensando en Diego. - I am in love. I am thinking of Diego.
Pienso que deberías salir con Cristina. Es una buena chica. - I think you should go out with Cristina. She is a good girl.
Dices que no me ames, pero no te creo porque tus ojos dicen otra cosa. - You are saying you do not love me, but I do not believe you, because your eyes are saying other thing.
Creo que ella te ama, solo tienes que ser paciente con ella. - I believe she loves you. You just have to be patient with her.
I hope it is clear now. Please, feel free to ask me if there is anything else I can help you.
Have a nice day.