My general advice is that if you have to ask, you shouldn’t go used. I buy and build a lot of used bikes frames and used components now but it took me a lot of year to be confident enough to do that. I’d suggest buying new for the first few bikes you own until you get a good knowledge of what makes a good bike. mcours2006’s advice is very good.

I do agree that you should go for a new bike if you are new to cycling.