how to access CSS HTML?

I have not had the chance to really monkey with any coding or anything since I was probably 11 years old on myspace when that first came out, since then I've long forgotten how to access the "editing" aspect of customizing my page, control panel? not sure what to call it. but I'm fairly confident that I can get the hang of it once I'm pointed in the right direction to access it :) thanks in advance for any advice! hope everyone's as psyched for this as I am.