Start with screen printing, and design and go from there. If you knock on doors M-F, or call and speak with the person responsible for advertising, management, purchasing etc, you'll get plenty of business.
Most companies already have their own artwork, they just need the t-shirts. Cheapest way to start is with a heat press and either use transfers you print or have a company like ACE transfers print them for you.
Go to a trade show to see whats on the floor, and meet vendors.
Whatever you can't do, outsource to contracting companies. You can still make a lot of money. Also, buy the equipment, when you've paid for it, it allows you to be competitive.
Don't be affraid to buy used equipment. Some people almost give it away, for different reasons. If you can afford to hold a second job until you can afford to pay for something do it.
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