Hi Jenna, Isofix is a term for a standard system for fitting child safety seats into vehicles in Europe. This ensures safety standards are met and means that manufacturers of seats and motor vehicles are singing off the same song sheet -
You can check whether your car has Isofix connectors fitted either by looking in your car's handbook or looking up your specific make, model and trim standard on a car review website like Parkers, then navigating to the 'equipment' page where Isofix will be listed if it's fitted.
I've put a link to the Toyota Auris (just a random choice) 'equipment' page where Isofix child seats are listed in the 'basic equipment' box so you can see what I mean -
As for which seat to choose, I'd read online reviews including the 'Which' review as theirs are from genuine research and customer feedback, then buy from a reputable supplier to make sure it isn't a cheap, unsafe knock-off made in some criminal enterprise sweatshop.
Good luck and congratulations on being a new mum. Once baby is safely installed in a nice car seat I'm sure you'll soon be back asking about the best car upholstery cleaners.