I am writhing a story! Should this boy in my story, go with old girlfriend or stay with his new girlfriend.

Think of the main theme of the story and try each way and see which fits in better to the main story line.

I read once that we should first create our characters, then just follow THEM through the story, they know what to do.

Take a look at this article on developing your characters before you start to write the story itself. It's good.

The Cast- Developing Your Cast Of Characters

I already have the characters

I'm referring to writing up your characters traits and habits and shortcomings and oddities, know ALL of that ahead of time. If your guy has been properly setup in advance, HE will know which he would prefer to do when faced with the new girl or the old girl.

Also, in writing, you are trying to create tension, then resolve it. If this is the beginning, or near the beginning of the story, the choice he makes should probably be the wrong one.

I have wrote stories before, if you did not know this is suppose to be a teen thing not that I am writing a story, that is why I put it here. I just wanted to what I should do about these characters in my story. I mean if you were these characters what would you do?

Aren't you a frustrating little one? Look, the answer is the same.

If you're writing a story, you already have your characters. Teen boy and 2 teen girls. Look at each of the characters you've created, decide which pairing (new girl or old girl) will create the most drama (meaning it ain't working out), and pair THOSE two. Now you have a story with somewhere to go.

If on the other hand you're NOT really writing a story and you're secretly asking about a real-life situation, always go with the new girl... as long as you're not still going with the old one. IF you are, be sure to officially end that first.

What happens if they have a child

Every grown up around them goes into "I told you so" mode.
College plans are canceled.
Both work night jobs to save money.
They renovate the room above the garage to live in.
Friends start to distance themselves and everybody gets depressed.
8 months into the pregnancy, the guy decides to go back to the old girl, and does.
Everyone is mad at everyone.
Ah... so much tension.

I know it is a complicating story. So you would say the guy should go back with the pregnant girlfriend?

In a story, he should be with the pregnant girl and then dump her so everyone hates him, makes for great tension in the story.

If this is REAL LIFE and not a story, men never abandon their babies, so he goes and stands by his new family for the rest of his life.

Well the other girl wants to start a family with the guy.

In which is another thing that makes this story juicy.