I don´t understand if you want to know the meaning of these sentences or just how we use them, but I'll try to explain both. When we say "cogerse un pedo" we mean that we are going to get drunk, but not a bit drunk rather a big one. "Ponerse" could be a synonym to say the same thing, but we often use it when you take other drugs apart from alcohol. "salir de juerga/marcha" means going out or going clubbing. I saw that your didn´t understand the meaning of the expession "esta noche salimos", this sentence means just that we are going out tonight. When we use the verb "salir" to talk about plans we do not use it to say that we are going to be outside home rather we use it to say that we are going clubbing or we are going to meet with someone and drink a little. I don't know if you understand what I mean.