Can u catch a std from sitting gon a toilet or from pee splashing up from one on to you??

Normally, STD's are not passed from toilet seat contact. Most are contracted from direct skin-to-skin contact, mucous membrane/ body fluid exchange, or blood route (HIV as example). I can tell you that pubic lice and pubic crabs can survive outside the body and on a host (such as the potty) for about 24 hours. This is still very unlikely.
What you are likely to get from a contaminated toilet seat (such as in a gym or a school) is staph. :) Everyone's favorite little bug. My suggestion? Put down the toilet seat cover or a couple sheets of toilet paper before sitting down, and this should protect you. :)

Well, my mother taught me to never say never... so I won't say "never"... however, your chances of catching an std via the toilet seat are VERY slim to none. Unless there's some sort of fluid (blood, mucous, etc) on the toilet seat, and you have some sort of an open wound RIGHT where you're about to sit down on, and the two make contact... then you MIGHT get an infection... MIGHT.

It's sanitary to put down the toilet seat cover... I usually carry a bottle of hand sanitizer with me and I usually just wipe the seat with a little bit of the hand sanitizer on the toilet paper.