Starting a print on demand store is a easy way to make merchandise without loads of upfront costs. You basically place your designs on products and another company makes them and sends them directly to the customers and you get a percent of the profit. You don't get as much profit as you would if you brought merch wholesale to start with but to do that that would mean spending loads before you even start selling and of course they will need to be stored too so these print on demand services are handy for start ups or someone who doesn't have the time or space to personally deal with the orders.
There are many companies who offer this service offering different products, commission percentage and locations so you will need you have a browse to find one that meet your needs.
I went with TeeMill. They are UK based and have the same values as me. They use organic cotton and send out their items plastic free. Even their factory is run on renewable energy. There are other companies that offer a wider range of products but their ethics really was the point that made me choose them. With them I also know where the product are coming from where others aren't as clear. Some of the larger names use different places for their printing all over the world and even though they say the mostly print in the same country the order comes from it also says some product can only be made in certain countries but are quite vague about which product so you might end up ordering and end up with surprise import charges if it comes from somewhere else.