What you are describing is totally normal. The bigger your boobs, the heavier they weigh. The perfectly pert boobs you see on TV, even the huge ones, are usually implants. But natural boobs are, you know, susceptible to gravity, and that is why you think they are 'saggy'.
Please do not worry about this. You are a woman now, with woman's boobs, and your boobs are acting exactly as they should. I know it's hard not to compare your body to others, esp. those online, but no woman's boobs can defy gravity, so remember that next time you start comparing your boobs to those you see online.
As for losing weight to reduce sagging, well, whenever I gain/lose weight, my boobs grow/get smaller. But losing weight simply because you want to shrink your boob 'skin' isn't really the way to go. You can always look up boob firming exercises online to strengthen the muscles in the hope of making the skin more taut, but you are worrying so much about nothing.
Listen, unless your boobs are sweeping the floor, you are fine! Seriously, though, you are 18 and have healthy boobs that are YOUR boobs and are 100% normal. If you are really concerned, please go and see your doctor and ask them for advice and they will tell you that you are perfect as you are. You're 18 and at the age where every girl starts looking at their body in horror and disgust. You could be Kim K right now and you'd still find something about your body/boobs you don't like, because that's how 18 year old girls' minds work, and it's exhausting. When you're 25 you'll look back and think "what was I even panicking about, my boobs were great".
And, remember: there are girls out there with very small boobs who would give anything to be bigger chested. Big boobed girls envy the small boobed, and vice versa. We always want what we don't have and think the opposite of ourselves is more attractive.
Your boobs are fire. FIRE.