Sort of like "Get your shit together." Means you need to fix something or get better at doing something. Only complete thugs/ballers can use this without sounding gay.
Guy A: "damn dude, I can't figure out how to get our internet to work."
Guy B: "Well you need to get your dick straight."
Guy X: "Damn I just messed up three serves in a row"
Guy Y: "Get your dick straight, bro."