I'm 13, girl. I accidentally touched my friend (boy)'s private part. He stared at me, but I pretend like nothing happen. Will he think something sexual about me now?

Well it was an accident he should know that . And if he thinks that way I would just stay away from him for a little.

He may have some sexual thought but you should clear it out by acting natural... And best of luck

He will probably think you are making a move on him. Watch out!

Make sure he knows it was an accident so its not all awkward

Hello friend, you do not need to worry. It is like a mere accident. And I am sure that, your friend (boy) has taken it sportingly. So don't get upset. 

You'll be fine just stay away for a little and then he'll for get

I think there is not so big issue. If you would have said sorry to him on the same time. I think the issue would have got solved on the same time after apologizing. But now its too late and if you apologize now then he will think that there is something in your mind and he will behave accordingly.

So now there is no need to say sorry, just behave like nothing was happened before and let the matter close.

I think you should just tell him it was an acident and be straight to the point about it. If you dont feel that way about him, he needs to know. Because you wont want anything funky going on if it is not what you want to happen lol. Best of luck, I hope this helps and things get straightened out :)

Just. Back like nothing happened if he asks you say I don't know what you're talking about.  

I did not realize I was talking to a 13 year old -thought this was for adults.  Please do not rely on my opinion.

Yes, he most likely got a boner and probably has had sexual thoughts about you. Pretend it never happened and continue talking to him like normal so things don't get weird or anything :)