How do I become a christian? I know it sounds a bit weird, but how do you start to believe in something you do not believe in? Can you give me any good reasons to believe in, or at least not reject, the existing of a God.

Can you perhaps suggest a book on the subject? Though the question is a bit weird, I'm quite sure I'm not the only one.
Believing in god can be a bit difficult as humans dont have all the information needed to make an informed decision about this topic. 

If i had to answer your queston id say that its up to each individual person to form their own opinion and find out what makes them feel true. 
You may want to try reading some religious books if thats what resonates with you. 

As for why someone would choose not to believe in gods existence, well that comes down to personal beliefs, values, and experiences. Some people might see belief in gods as childish, unnecessary, or inconsistent with scientific knowledge. Others might argue that gods exist but choose not to reveal themselves to humanity because they prefer to remain mysterious or have control over their followers. Some people might also hold philosophical objections tothe concept of gods, such as arguments against divine command theory or belief in non-existence of universals.