I know that for many people sex is sort of free and casual. Unfortunately I found the hard way that there is no such thing. There is always an emotional attachment and a certain amount of pain if things do not work. I used to think my body was my own and I could do as I like. I managed to get myself hurt, get pregnant, have a miscarriage, get married in a dysfunctional relationship, and have that marriage fall apart. I can't imagine the heartache I could have saved my self if I had controlled my hormones as a teenager. As girls we often are looking for love and enamored by the first guy who shows us any attention or affection. It is hard and often our hormones rage.
It is hard to give you any real advice other than to say, I have many regrets from my days of experimentation. I finally put my life back together and I am now married to someone who respects me, loves me and always is there to satisfy my needs. I would make that your goal. Take your time to be discerning and if a boy wants to treat you like another conquest, run as fast as you can