Like someone said up thread it's worth checking your car has a spare wheel in the first place.
Usually it's in the boot under the carpet, but sometimes in a different place.
But lots of modern cars don't have spare wheels at all though in which case you may have some puncture repair gunk that's designed to just get you to the nearest garage so it can be replaced.
Or some cars have run flat tyres (some Minis, some BMWs etc), so if there's a hole in it you'll still be able to drive on it to get you to a garage.
Random Tips
If you have a spare, there should be a small jack in the boot too. You'll probably have a locking wheel nut somewhere in your car too.
Make sure you find the correct jacking point under the car, it usually looks a bit thicker/more solid and certainly on our car there's a small triangle/arrow to indicate where the jack should go.
You'll be best off parked on something solid too - not shingle or a soft verge.
The tyre size is written on the sidewall along with the make. You'll need this to order any replacement.
Most cars have the same size front and rear but some cars don't. Always worth checking.
It will also say whether it is a run flat tyre there too if you're not sure - only some models of some car brands fit run flats as standard eg. Some minis/BMWs from what I can recall.
If in doubt check with your local garage.