“You've just got to force yourself to sit down and concentrate.”
Your son is getting bad grades in school and you want him to study harder. He complains that he can't focus on his homework and wants to know how he can become more focused. You say this because you don't think there's any special method except just trying harder.
You've just got to force yourself to sit down and concentrate.
To "force" someone to do something means to make them do it. You can use this word somewhat playfully:
They forced me to sit there and watch it.
Or you can use "force" to talk about something violent and frightening:
They forced everyone at gunpoint to hand over their money, phones, and other valuables.
You can also "force" yourself to do something.
I forced myself to sit there in front of the computer and keep working on it until it was done.
This means to use your willpower to make yourself do something that you don't really want to do.
In English, "sitting down" is associated with doing something seriously. So "sit down and ___" means to do something carefully and deliberately:
The word "concentrate" works by itself; you can't say "concentrate (something)". If you want to express the object of someone's concentration, use "concentrate on (something):
Can you turn the TV down? I need to concentrate on this homework.