It is very important to identify who your customers are going to be. If you specialize in networking, you don't want people calling you for websites. Always remember that a happy customer tells 4 people, and a disgruntled one tells eleven. Sometimes you have to eat your profit or take a loss to protect your reputation, because your word has to count for something. Learn to recognize customers you don't want. I spent 10 minutes on the phone with a man the other day who insists that his "software is bad", but he couldn't tell you what application is giving him trouble, and his real problem is that he has malware. Some customers you simply don't want.
Others you do- I ran a custom leatherworking shop for 15 years, and I made a good living, but no one was as eager to lay down cash as the folks who can't get online, or think they lost their family pictures. A client's desperation can be proportional to your fee, especially if you need to drop everything else that you are doing. I do tech support for grandmothers from my church. They don't pay what businessmen do, but they are super sweet to me, think I'm a genius and feed me home made cookies. Some fringe benefits are not as tangible as the IRS would have you believe. Once you know your market, cater to them. Make things easy for them. read the manual so that you know what you are doing and don't waste your time. If you agree to fix a computer, have a description of what you intend to do and the applicable charges, and have the customer sign it so you know they agree with your plan. You need a "hold harmless" disclaimer in there, too, because they won't understand your reasonable explanation if something goes wrong, no matter whose fault it was. This contract will CYA if something goes wrong. Negotiation is optional, but keep in mind, do you argue with the plumber?
You can generate goodwill by providing basic education when possible, and the added benefit is less dumb phone calls while you are in the middle of something else. If you doubt whether they can pay, skip the job. It's not worth it. Your biggest asset is being able to make smart diagnostic decisions without wasting any time. If you need to brush up on your skills, consider community college courses or other continuing education. Time is money. If someone calls you with a job you don't want to do, or you know you aren't prepared for it, decline it. Giving out the name of a better qualified colleague will earn you respect, and save you aggravation. Advertise in the phone book, signs, flyers, local newspapers, and pass out your card to get the word out to your community. There is a man running a shop in my village, and he is so busy he has waiting lists, and I kid you not, he didn't know the difference between an AMD processor and an Intel when a friend and I visited his shop and quizzed him. None of my neighbors would know, either, but that's not the point. Don't even think about becoming successful until you are confident that you know your stuff.
It is cheaper not to have a storefront, and in computer repair/building/sites you can get away with that. If you don't operate a cash business, you may qualify for tax reductions on portions of your home expenses. See an accountant to get the best advice for your situation. If you aren't doing much volume, it might be cheaper to call your venture a "hobby". If you are more than that, see a financial planner for advice to work out your goals, and how it benefits you best to set your business up. You cannot go wrong writing the whole proposal on paper, so that you can get professional advice without confusing your expert. When figuring overhead, in addition to inventory, you need to take into account insurance, taxes, utilities, hardware/software expenses, advertising, and your time. If expenses add up to more income than you project, and you don't have investors or a trust fun, consider reworking your plan until it's achievable.
People never fail unless they try. And, you cannot fail if you learn something for the next time. People who never fail are boring and have no courage. Best of luck to you!