"I'm on.." "I'm up.." in American englis

Ok, I heard a lot of these expressions, and I'd like to know their correct usage in American english:

I'm on for something
I'm on to something
I'm up to something
I'm up for something

Which is correct? Can u show me some examples about the meaning?

I'm on for something -- I'm planning/intending to do something -- "Are you still on for dinner tonight?"

I'm on to something -- I'm aware of, or in the process of figuring out -- "I know what you do when you leave here -- I'm on to you."

I'm up to something -- I'm doing something sneaky or nefarious -- "He's got that smile again; that means he's up to something."

I'm up for something -- I'm willing or ready to do something -- "Who's up for going to the beach today?"

thanks uriel, very clear :)
Another question though..

When I say I'm on to, I have to use the "ing"form or the infinitive?(I'm on to go... or I'm on to going)? and for I'm up to?

With the "I'm on to..." construction, you basically translate it as "I'm aware of ..." or "I understand what...." and you generally use it with a noun or pronoun, not a verb. "I'm up to..." is similar. Those terms already contain your main verb, so to speak.

With the other two, you can use any verb form, and even nouns -- "I'm up for a trip to the beach" or " I was up for a swim earlier, but I don't feel like it now."

Ok, but, for example , if I say:

I'm up to hit on some girls

Is that correct? or I have to say I'm up to hitting on some girls?

As I understand the meaning of "I'm up for hitting on some girls is different..

Both mean the same thing, that you are interested in getting a girl in an amorous situation.

"I'm up FOR hitting on some girls" is the way I'd go (if I went that way at all...)

"I'm up TO hitting on girls" doesn't work for me idiomatically. That's more for things like "I'm up to no good, I'm up to my ears in debt, I've had it up to here with your crap, I'm up to bat" -- don't use "up TO" with a verb the way you're proposing; it doesn't sound right.