Don't worry, you can definitely fit car seats into small cars 
Isofix is sort of like a socket you can plug the car seat into, to attach it to the car without using the seatbelt. It was invented because people often make mistakes when using the seatbelt to attach car seats - isofix is much easier.
If you push your hand down into the back seats of your car where the backrest cushion and the seat cushion meet, you'll feel some little square metal loops - those are the isofix brackets.
All cars since 2013 have had isofix and most cars since 2006 have had it, so it's likely that your car does have this unless it's older.
For the first stage, most people like to have a car seat that they can carry around and attach to a pram. These last up to about 12-18 months. For these, if you want to use isofix to fit, they normally have a base available to buy. You fit the base using isofix, and then you can just press a button to lift the car seat on and off the base, which is much quicker than fitting it using the seatbelt. This works well for 3-door cars as you don't have to climb right into the back, you can just lift the seat in and out.
The other option for a 3-door car is to disable the passenger airbag and fit a car seat using seatbelt on the front seat. Sometimes you'll even have isofix on the front seat, though this is rare. You MUST disable the airbag when using a rear facing baby seat - this is law because children and babies have been killed by airbags.
It is also possible to buy a longer-lasting car seat which stays in the car all the time, but this is a bit of a pain with a 3-door car, because babies have to be rear facing at least until they are 15 months old (preferably longer) so it can be tricky to get them in and out of the seat like this. Also these seats don't usually give a very good fit for newborns. So it makes sense for most people to opt for the "infant carrier" type seat first.
If you go to a shop that sells different baby seats they will normally let you try them in the car for size and to check how practical they are to get in and out which will be even more important if you have a small car. John Lewis is a good chain to go to, or if you have an independent nursery specialist near you, they tend to be even better. If you want to give a rough area I can have a look for you.