Just from my own point of view, gossip about others, and tall tales/embellishment about oneself are generally the most engaged in, and worst form of wrong speech I am likely to do or be around. This was double or triple as a teenager than as a middle aged person.
I might cuss every now and then, but that is fairly mild in my view compared to these two things, which seem to be a common part of much friendly conversation.
Especially at your age, personally I would start with these rather than cussing, these are more likely to bring direct harm to you and others. Not saying don't avoid the cussing, it's a good thing to avoid unpleasant terms..but it's very superficial in comparison to trying to eliminate these IMO. Also learning to tell the truth more can transform you in some big ways that just avoiding bad words cannot.
It's very hard to avoid doing this sometimes, I find that usually such conversations begin with one person asserting something they did was "better" than the person being gossiped about. If this is the case, it can be effective to pull the conversation away from the people involved to start with, and just talk about the 'wrong thing' in question. It still might not be a great conversation, but as far as malice goes I think sometimes it can diffuse things.
For avoiding self-aggrandizing talk, just try not to do it...surprisingly i've found you will be more liked, rather than less when you engage in fewer stories about yourself. Asking people honestly about their opinions or experiences to point out substantive things can help...think of it like The Socratic Method For Teenage Conversations
A bit from the Dhammapada that I try to reflect on when I get pulled into speech I don't like, among other things:
Evil is done by oneself by oneself is one defiled.
Evil is left undone by oneself
by oneself is one cleansed.
Purity & impurity are one's own doing. No one purifies another. No other purifies one.