I'm 18 too and it seems that impulsivity, according to others, is higher now then it will be later. I sometimes act on impulse, sometimes think things through, depends exactly on what we're dealing with here.

Ask yourself, what do you want the outcome to be in the situation? Next, try to rationalize what you want to do; will it lead to that outcome? Will it be the desired A-->B or is it going to be some complex A-->C-->F-->G-->B?

Such as if you're going to ask out a hot girl/guy. Desired outcome is meet and know them very briefly, and hopefully allow more knowledge to be obtained from them later. If you're going to go up and say something like "Hey, wanna have sex?", that's not going to work, even if it is what you want to be a later outcome. Think what to properly say.