Have safe sex. You hear this message everywhere you go. Schools are moving from promoting abstinence to teaching students how to use condoms. This is where it all starts. Use a condom every time. No matter what.
Here is why you should use a condom every time you have sex. You are going to have sexual intercourse with someone who has probably had sexual intercourse with their fair share of partners. They have put their penis (have have their vagina inserted) by others, so when you have sex with someone, it's like having sex with all their partners too. If any sexually transmitted diseases have been left behind, you will catch them too.
Think about it exponentially, if you have sex with one person who has sex with 2 people, those people had sex with 2 people and so on. 2 sexual partners quickly and easily turn into hundreds of sexual partners. You see how easily it can be to transmit sexual diseases? Always use a condom. Hopefully, thinking about sex in mathematical perspectives will scare you into using condoms.
If you are unsure about how to put on a condom, then check out this visual aid so you know. It's important to use a condom properly. Also use lubricant because it will prevent any friction and reduce the chance that the condom breaks.