I really want to skip 8th grade and move on to high school because I am basically the oldest. I flunk 3rd grade but science than my grades have been pretty good except for 1 report card I got a 60 in one class and 70s ,but I ussually get 80s and 90s even 100s. I think I was going through a really hard time understanding the lesson when my grades went down, but I brought them back up very quickly because I felt really bad.
Please tell me how to skip a grade I would do anything.

Talk to your teachers. Only they can decide if skipping a grade is suitable for you. From the sound of it, I doubt it. Someone has to be the oldest, that's not a reason for skipping. The best reason would be if you were so far in advance of the rest of your class that the world would be extremely boring to you. If you were scoring 100s almost all the time, then you have a good argument for skipping. 70, 80 and 90 sound like you are in the right class.

Start getting A's. After the report card comes out make an appointment with the principal or your counselor.
It will be a lot easier if you have A's than if you have B's or C's

Yes, start getting nothing less than 95's in everything and at the end of year testing ( CAT or what ever is used in your state) show you are testing at a level higher than the grade you want to pass.