The answer?
Brag.
It seems almost a universal habit that if you are a rapper and don't know what to rap about, you should start rapping about what a badass you are.
On the other hand (not that I've done much looking, but) poems where the narrator is the poet or poet's persona actively bragging about him- or herself seem relatively rare. There seems to be this idea that bragging is unseemly or dilutes the purity of the art.
I say screw that. Today, if any of you humor me, I challenge you to write (whatever genre and prose/poetry/script whatever) a piece bragging about or up-talking yourself. It doesn't have to feel true to you. You can adopt a persona that does feel this way about yourself, even if down at your core you truly don't. I encourage you to go over-the-top with this.
Have fun with it.
To inspire you, I leave you with Eminem, from GOAT (but really I could give you Eminem lyrics about how he's so great ALL DAYYYY, because that's how much he talks about it):
....
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
Reflecting on how I was pretty good with Zangief
And even better with Chun-Li,
In the original Street Fighter.
Sorry. :)