Obtaining a steady client base of contract printing requires a lot of dedication, customer service, quality work, and understanding of your capabilities. There isn't an "on" switch that you can just throw and work flows to you like a river.
Your first step is going to be essentially understanding the qualities that you bring to the table and can offer someone. I'd start with a robust business plan. There are tons of websites and books dedicated to this, so I won't belabor the point, but this should be your primary focus when starting out.
Contract printing is based on handling the work for someone else, knowing that they still need to mark up the order to make money. That being said, the margins are quite low and people will leave you for a penny or a nickel less than the other guy. The only way to do it right is to understand completely EXACTLY how much money every step of the process takes, and where you can add more money to your margin in that process.
Large contract printers have highly automated and efficient shops. I should know, I run one in Milwaukee - Visual Impressions. Take a look at my Pinterest board for some pics of the shop to get a glimpse of what I mean - Behind the Curtain at a T-shirt Shop
Not saying you can't get into this market, as there's plenty of work to go around. However, you need to make sure you comprehend all the facets of this market before you set yourself up for failure. Don't compete on price - instead offer the intangibles that are going to be difficult to ignore: Excellent customer service, outstanding artwork, able to hustle and do things that other shops can't or won't. Build your stable of clients one at a time and knock their socks off.