If you have the time, energy and ability to learn how to do it yourself, then I would say, DEFINITELY do it yourself.
In fact, I would suggest that you *make time* to market your company and find new and effective ways to do it on an ongoing basis.
If you come to the point where you have a lot of funding and not enough time to handle the day to day marketing of your business, then at that time, it might make sense to research outsourcing the marketing to a firm.
Simplified, you just have to get your product in front of where your potential buyers are. For heavyweight blanks, that would mostly an urban market or businesses that cater to urban markets (I would think). You could attend tradeshows where t-shirt printers gather (like the upcoming
ISS Tradeshow in Long Beach, CA). You could attend other sportswear shows like ASR or Magic to get in front of buyers. You could get right down to the store buyers in urban areas and visit local corner stores to find out their buying process.
You probably know more about your product and customers than a company you would outsource to. So I would start with you doing it and then once you've got it down or know your shortcomings, you will be more prepared to hire someone to do what you can't do or don't want to do.
should we handle marketing ourselves (guerilla) or hire a brand management/marketing company? 
