I like the idea of practicing on sheets of plasterboard. Just screw them up against a wall in your garage or similar and try with some freebie tiles from Freecycle and cheap/free adhesive. You could also ask for this on Freecycle. I just gave away a load of adhesive and grout that was only ever good for kitchen splashbacks.
That kind of set up is used at tile training courses.
One of the biggest challenges you'll face when you actually do a room is the unevenness of the walls. Not only that the corners may not be plumb, but also that solid and some stud walls are often not level, so levelling out is key. A number of tilers here are also plasterers, so they have a head start IMO.
If you do paid work for people or even as favours, stick to kitchen splashbacks and simple tile around baths etc.
From job to job, research your adhesive and grout to start with. Tile suppliers are absolutely evil in selling you the wrong adhesive for the job. Come and ask here what you need, then go and buy it.
Also, if you want experience and you can't get paid work, ask about volunteering. There will probably be groups like help the aged who would be glad of your help in this way.
I am a member of a
community group that have an old building that needs maintenance. I wanted a bit of plastering practice - hadn't done any for a while, when part of a ceiling came down. I got my practice and the building got its ceiling repaired FOC. Actually it looks very good even though I say so myself.
I too am going to look at a course and have an offer of working with someone who is near me, which I will take up.