Do you have to be a junior/senior to take upper division courses? If not, do I just get to sign up for them or do I have to talk to someone specifically
If the prereqs say “upper division standing”, then you’ll need to have enough units to be considered a junior or senior. Similarly, if the prereqs specify a specific unit amount then you’ll need at least that many units.
Other than that, you can sign up for upper division classes whenever you want as long as you’ve taken the prereqs. Though if you’re a freshman, it’s probably a good idea to talk to the professor to see if you’re currently prepared to take it or if you should get a little more college experience under your belt before taking that specific class. I’ve had a few upper division classes with light prereqs explicitly say in the first class that the class is absolutely not intended for freshmen despite what the prereqs may indicate. If they don’t explicitly say that, ask to make sure.