I want to know the best way to approach a retailer that i want to carry my line of shirts.
You need to focus on your brand first. No one wants to carry a brand no one has heard of and has no brand awareness.
There are tradeshows where retailers go called magic tradeshow and pool tradeshow.
Also just send a friendly email or give them a phone call. They might not want to stock your brand but its important to make connections so it will be easier later on.