How do I express what I think?

You need to use the neutral form and to add at the end of your sentence と思う - とおもうeg. 明日雨が降ると思うNeutral form means that when using- a noun, you need to add だ at the end of the noun eg. それがバッグだと思う- an adjective, you use the normal form for い-adj eg. かわいいと思う; and add だ for な-adj eg. 簡単だと思う- a verb, you use the dictionary form