To not have sexual intercourse.
This is the only way to ensure that you will not get a disease through sexual contact. But remember, many diseases that are classified under STD can also be transmitted by using unsterilized hypodermic needles when taking drugs. Furthermore, there are drugs such as Rohypnol that are mixed with alcohol, in order to incapacitates a victim, making it impossible for them to resist sexual assault. Other drugs can cloud your judgement and allow you to break an allegance to abstinence. So the best way to avoid STD is to abstain from sex and to abstain from drugs, including alcohol.

Abstinence is also the best way to avoid a pregnancy. No birth control is 100% effective. Using two forms of birth control such as the pill and a condom with spermicide is very, very close, but only if the birth control is used properly. If a woman skips a dose of the pill, she is not using her birth control properly, and its effectiveness is reduced. No man has any guarantee that a woman will take the pill as she was instructed. Likewise, no woman has any guarantee that the man won't misuse his condom. If two people use a condom without any natural or synthetic sexual lubrication, the condom could become too dry and break. If a man uses a condom, drains it, then uses it again, he is certainly not using the condom properly and it will definitely not prevent a pregnancy. If a man withdraws his penis from the vagina, he doesn't hold on to the condom and the condom slips off into the vagina, it will not be effective against pregnancy. These things can happen. If both partners are educated, they can do their best to make sure these things don't happen. However the only 100% effective way to prevent a pregnancy is to abstain from sex and to not let the tip of the penis or semen come in contact with the vagina.

People do abstain from sex. It is not impossible. Your partner may make you feel like you're asking too much from them. If you feel strongly that you don't want to have sex, and your partner feels strongly that you should have sex, then you obviously have a major difference in opinion. Differences in opinion are not a bad thing. In fact different opinions make this world a more interesting place to live. Neither is debating the issue and discussing the pros and cons a bad thing. This gives you a chance to express your feelings on the subject. It is natural for most people to desire sex. However, if you have clearly expressed your choice to not have sex, and your partner continues to pressure you until you feel uncomfortable, then you should consider the possibility that you and your partner are not compatible. If your partner wants to have sex so bad, then they should feel free to end the relationship with you and have sex with someone else. If you feel that you want to wait, then you have the right, and if they can't handle it you may consider letting them go. There are tons of people in this world that are around the same age as you are. And it is highly possible that you will find someone with the same values and someone who has made the same decision about sex.