How to get started with IT and Networking business?

The best way is to, well, get qualifications. Any old bod can put flyers through your door but someone with qualifications will be able to prove their technical knowledge, rather than just talk about it. For instance, I can go into a corner shop and buy 20 pints of their cheapest cider, but without proof of age there's a chance I wouldn't be served.

If you're doing networking, then I presume that it'll be for business (home networks don't require an IQ above 50 to setup and anyone doing custom networks will do it themselves 90% of the time as they already know this stuff), and people in business like to see qualifications, or at least an established customer base.

It's certainly possible, but a lot harder. Start with close family and friends, ask them to spread word around, some s*** is going to stick sooner or later. As you say, put up ads and flyer around but don't be disheartened if you don't hear anything. Put something that nobody in the area might have or can offer, something like a "No fix, no fee!" kinda thing, which you can afford (as I'm taking you're unemployed) compared to if you were working 9-5 (all free time is precious).

If you've only built one computer though, stay away. These are people's machines we're talking about here, they may "only" cost £300 or something for the budget computers, but that's still £300 to these people, so screwing up a machine with an amateur move like not taking static away from yourself while fiddling around with the inner parts is going to get you in real trouble. Go and work for someone first, go into a PC repair shop or something and just talk to the owner about working in that sector, they'll know a lot more seeing as, well, that's what their job is!

I really wouldn't recommend going into this sector after a single computer build, no qualifications and by the looks of it, not a huge amount of confidence in the idea (you would have gone out and done it already, rather than posting on a forum). Go and do work for someone else first, whether that's a big chain or a smaller, one off company (usually the best place), they might even pay for your qualification training and you'll at least meet people that might prove fantastic contacts later on in life.