I'm a beginner and im thinking of getting thicker gauge strings and a guitar store employee told me that if the thicker string doesn't fit around the nut I'll have to go and get my guitar a setup. If I get strings with thicker gauge that does fit without getting a setup is there any potential damage to my guitar?
Thicker strings engage more force. I've heard of a case where the guitarist changed strings to thicker ones, and the neck couldn't support the force needed and hence doubled.
I suggest you take your guitar to a luthier who can inspect it and advise you on your specific case (guitar/string gauge).
Hope this helps!