The word omou (思う) means to think. This verb is used for your thoughts, feelings and opinions. The verb kangaeru (考える) also means to think or to conceder, except it refers to the action of thinking.
What do you think of her = kanojo o dou omoimasu ka
This book is good I think = kono hon wa ii to omou
I am thinking about my family = kazoku no koto wo kangaeteru
Very often you will see the verb omou used after the particle to. When you use this it means that one is expressing their thoughts about something, or that they are unsure about something.
Kim eats apples I think = kimusan wa ringo o taberu to omou
Tomorrow will rain I think = ashita wa ame ga furu to omou
Takashi is cute I think = takashikun wa kawaii to omoimasu
You can also use this verb in different forms of verbs and with different verb tences.
I thought it would snow today = kyou yuki ga furu to omoimashita
I don't think the kid wants to go shopping = kodomo wa kaimono ni ikitai to omoimasen (子どもは買い物に行きたいと思いません)