how likely is it to contract hiv or something from....?

The likelihood of you getting HIV from the scenario you described is essentially zero. The most likely way to get HIV is from the blood of someone who is infected, usually through unprotected sex, especially anal sex.

Many people are concerned about using public toilets because of the germs they might come in contact with.

It's unlikely to catch any disease from skin to toilet seat contact, especially if you don't have any open cuts or sores or wounds. Skin acts as a pretty effective barrier to disease. Be especially careful if you do have any cuts or breaks in the skin around your fingernails.

There is absolutely no chance that you have contracted HIV from this. HIV is spread by four and only four bodily fluids: Blood, semen, vaginal fluids, and breast milk. If none of these fluids have come into contact with a mucus membrane or area of your body that could be a gateway to your bloodstream then contracting HIV can be 100% ruled out.

You have nothing to worry about. I've studied infectious disease in and out during grad school. The worst case scenario, you manage to get crabs, and that is highly highly unlikely.

You can't get HIV from a toilet seat. You can get crabs( pubic lice) tho