Hi,
Firstly don't feel bad for not knowing, neither did anyone else until they found out! My stepdad showed me all of this sort of thing when I learnt to drive but I have plenty of friends who wouldn't know.
I'm probably duplicating what everyone has said anyway but -
Find the correct pressure from sticker on door rim (mine is passenger side on the bottom, the rim on the main car body that you only see when the door is open) , or from your manual
The machines are usually either free or 50p, I'd have a couple of 50p's so you don't feel rushed and can make sure you do all of them
Remove your black caps all round and put in your pocket
Set the tyre pressure it will ask for BAR or PSI and you put the figure up or down with the plus/minus button
Money in and you will hear it switch on
Take the attachment, looks a bit like a long metal pipe attached to a hose, the end has kind of a hole on it, you fit that snug over your tyre valve and when it's in the right position you will hear it inflating your tyre. The machine will beep when it's reached the correct pressure so you move to the next one, you don't have to be able to see the machine
The hose might not feel long enough you have to give it a good pull round to get round the far side. I do my front two with it pulled over the bonnet end then pull it round the back way for the back two...if that makes any sense. If you need to pull your car a bit closer just do that, you see plenty of people having to adjust a little bit
Once you've been all the way around don't forget to put all your caps back on (I've found a full set on the ground before when doing my tyres at the garage someone must have just headed straight off and forgot them!)
I know you can maybe get it done for you in different places but, I think it's one of those things that it's just good to know how to do. Also, it affects how your tyres wear and how they grip the road so if it's important to keep a check on the pressure and I think you'll do it more regular if you can pop and do it yourself rather than have to go somewhere for it done.
I'd second watching a you tube video if poss, it might just help you not feel nervous of going the first time, then once you've done it once I'm sure you will be fine!