I dont think i am completely understanding what a dream sign is. I have plenty of reoccuring themes in my dreams such as my friends, driving, or doing school work, but these are all things i do or am around in real life.
Are these dream signs?
Do dream signs have to be things that dont occur IRL?
No dream signs do not have to be things that do not occur IRL. 
Dream signs are anything that happens very offen in your dreams that you look for and use to become lucid. One of the easyest methods to use dream signs to become lucid is to do an RC every time you see the dream sign IRL when it is one that happens IRL. Then you get in the habit of doing an RC every time you see it, and when you see it in a dream you do an RC and become lucid. This does require that the RC is not done mindlessly, but with the intent to realy question reality. Other then that, it can be a good way to become lucid. 
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Dream signs can be anything you want them to be.
In my case, I have a lot of tornado dreams so that is my dream sign. For the things that you do commonly in waking life that might not naturally trigger lucidity when you come across them do RCs.
When doing your homework do RCs, question whether you are really doing homework or dreaming you are. Then when you are dreaming and eventually RC during dreaming about homework you have a good chance of triggering lucidity.
Alternatively, you could try telling yourself while awake that when you are dreaming you will see “object” and when you see it you will know you are dreaming.
I have dreams about being in school. This is an IRL dream sign of course. 
What the others said. In fact, there are two ways of choosing your dreamsigns: first, find what’s repetitive about your dreams. If you dream about parks a lot (regardless of whether or not you go to parks a lot in real life), you could set the general theme “parks” as a trigger for RCing. So the next time you hear about a park, go to one or see a picture of one in the newspapers, do an RC.
The other is to find what’s abundant in your everyday life. You go to school? Lovely, start RCing whenever the theme school is brought up or you realize you are in school. If school happens a lot in your life, then it’s bound to show up in a dream, and by extension, if you RC in waking life whenever you are in school, talk about school or hear about school, you’ll end up RCing in the dream too.
I don’t have any dream signs… I just understand I’m dreaming when I see something… but every time it’s different! 
My dream signs have been various things. The first ever dream sign that made me realize I was asleep, was my sisters hair was way shorter then it should be. My most recent lucid dream was that my neck was ‘broken’ so I couldnt hold my head up and fell on the floor, and a dog I USED to own was lickin my face. But sometimes dream signs are simply a wierd feeling, nothin looks wrong but you feel funny and realize you’re dreaming.
I have sort of a related question. Do the dreamsigns you have need to come from dreams you’ve had before? Can you designate a dreamsign ahead of time? I think this is one of those things that’s given me a little trouble before too. Sometimes I’ve heard that if you’re replaying a just-concluded non-lucid dream as part of a WILD or WBTB , you pick out an image from the dream to use as a dreamsign, with the intention of recognizing it as such the next time you’re in the dream.
Part of the problem I have is that in dreams I’ve journalled, I’m having a hard time finding a thematic element that I can point to and say, “that’s my dreamsign,” and the ones that I can maybe point to are only thematic in the broadest sense, e.g. a person I know shows up in a place where I’ve never seen them before, such as a co-worker appearing in the church I grew up in. That’s the most recent example I can come up with.