...that it really would have been worth it to have waited for the right person. But since I couldn't, here are the things I would have included in my advice.
Use a condom
I've had the very fortunate (feel the sarcasm please) experience of having my first sexual encounter, and the next few until the following year or so, unprotected. Luckily, I've done it only with one person in that length of time, but as we all know, I don't know who else he's doing it with. A FuBu, the shortened term in the Philippines for Fuck Buddy
(All the benefits of being in a relationship minus the bullshit like not doing enough for Valentine's Day or his/her birthday, not spending 3 months salary on a stupid ring, and not spending enough quality time with him/her. - Urban Dictionary)
...taught me all about what I know, and he was a few years older than me. I was taught a lot and the education happened from my room, to his garage, all the way to cheap and sleazy motels. All without rubber.
Know the person
It's not always a perfect match when you're about to do the deed. Yes, he might be hot. Yes, he might be all that. But sexually, you might just end up blowing him with all your skills and he leaves you for shit without so much as a decent blowjob. Casual sex would only go so far, but making love with someone who's willing to do something to make you feel good as much as you would is really the entire point of it all. But hey, youth pretty much don't give a shit about the details, it's just sex. I'm just really getting old so I'm a bit more picky.
The internet isn't your safety blanket
I know it provided and still provides me with endless supply of porn and quickies, but I should have been careful. Good that I wasn't one of the more unlucky ones, but recently the internet has become quite unsafe with regards to sexual meet-ups. People have been robbed, killed and the most common of all, infected with sexually transmitted infections and worse HIV. Instead of venting frustration or de-stressing on porn, or finding a good time cruising sites, I could have just opened a book and finished more novels. It's hard to fight the hormones of youth, but at least I could have tried. I wouldn't have been so dependent on the internet.
Finally, I would have shared this simple thought to the 18 year old me: Love yourself. That pretty much covers everything that I could have missed out. Like the government now is exemplifying, a young person would have a hard time finding advice from adults about sexual health and sexuality and how one should explore it. The RH Bill is still being questioned, the church is trying to teach about sex. It'd really be confusing to anyone. So I hope, this reaches out to whomever needs it.
Always take care of yourself, you're the only one you've got.