The same way you went through school or learned to walk: Take the first step and start the learning process from the beginning of basics and progress through OJT (on the job training). The best way to learn is to get your hands dirty and remember that you have two ears and one mouth; use them in that order and you will do fine. For a beginner at any job the fatal flaw to learning is not asking questions; if you don't know the answer the only stupid question is the one not asked. Contractors don't like to hire green helpers because it takes time to train people to be safe (for themselves and those working around them) and the ABC's (terminology) of the trade, etc. I only know what worked for me in the electrical trade.When I was 16 I offered to work for free in order to learn the trade well enough to be productive and earn the job through my knowledge and fastidious labor attitude. As it turned out the more I listened and the quicker I moved the faster I reached my goal. The bonus was that my starting pay was the same as an entry level journeyman where three months earlier I was clue-less as to the difference between a 1/2" connectors, couplings, nipples, or EMT.
You will start by marking out your target market clearly. For example, a viable market would be women and elderly home owners who often need help with seasonal home maintenance tune-ups. Team up with a specialist in a slightly different area like a carpenter. It is best to set up your own firm that offers professional services to so as to tap into occasional clients.