Start by offering a pair of hands to friends and family.
When they move, help with the plumbing in, the wardrobes, flat packs, decorating, deep cleaning. When they have a project, be noisy- see how it's being done. Take on basic stuff pre trades such as preparing areas, emptying rooms, ripping out old furniture, bathrooms, kitchens.
Don't reach for the skies day one, know your limits.
Build you skills from doing straight forward stuff, learn what your weaknesses are and check out adult education courses (plastering, wood work, tiling etc) to address them.
You could trawl local adverts and see if any handyman, roofers, builders, decorators need a 'boy' and offer yourself as another pair of hands. Your more likely to get work from trade if you are available at short notice, can listen, are happy for modest pay (on the understanding you are on job learning), keen and most importantly bring value to the job.