I am not sure I want to get married yet. I am 18 years old. I love him and know that someday in the future he will be the one I will marry. I don't know if am being childish or if I should marry him now because I am going to marry him eventually. As I understand your question, you are a teenager who, after going with a guy for two months, is thinking about making a life decision about marriage.
Some people take more time than that to decide on what type of car they are going to buy.
Two months is not a very long time to know someone, much less knowing enough to make a decision that will affect the rest of your life. Rather than thinking you are childish for wanting to wait, I think that's a sign of maturity. Marriage at the age of 18 when you are expected to live until to you into your 80's is a long commitment. What you and your boyfriend have in common today, may be totally different six years from now when you are 24. The next seven years of your life will be a period of flux as you move from your teenage years into adulthood.