Sexually Transmitted Infections (STIs) can be passed on between people very easily. That's because there are many different ways they can enter the body:
1) Through penis in vagina and/or penis in anus sex
2) Through genital to genital touching
3) Through skin to skin touching
4) Through oral sex (mouth on vagina/mouth on penis/mouth on anus)
The skin on the genitals is very delicate, and any cuts or splits in the skin can be the perfect place for an infection to enter the body. It's important not to over-wash this delicate skin or use any products with fragrance. Take care if/when undergoing any hair removal as well, as it's important to avoid any small cuts to the skin.
Because most STIs don't have symptoms, you can't always know if you've got one or even passed it on. This is why testing regularly is so important. Just because you don't have symptoms, doesn't mean you can't have an STI. With so many ways for them to enter the body, it's important to have all sites of sexual contact tested at least every 12 months (but ideally 3-6 monthly!)
So, what's the best way to prevent sexually transmitted infections? Simple! Just use an external condom on the penis. There are also dams that can protect genitals on the outside.
Stay safe and have fun!