There are many websites now that allow you to have custom t-shirts made. This means that you will be able to design one and then have it made up. They work in slightly different ways and therefore you can make money from them differently.
Made to Order/Print on demand
If you have them made to order then what this means is that they will just make the amount that are required by you or your customers. There are sites where you can set up your online shop with your designs in it and you can direct your customers to it. They will then order the ones they like and they will be made up and sent directly to them. This can be hassle free for you, but you will have to pay commission on sales. The price of the individual shirts can be quite expensive as well because they will only be able to make them one at a time. However, it also saves you having to invest money in stock. People may also have used the company before and trust them more than perhaps they would buying directly from you, if they have not heard of you.
Batch Making
You could order a batch of t-shirts and then sell them yourself. You may want to set up your own website to sell them, sell them in a shop or use an online retailer to sell through. Having a batch made up will mean that each t-shirt will be cheaper per shirt but you will need to sell them in order to make back the money that you have paid to have them done. You will also have to consider what sizes to have made up and how many of each. This could be quite a considerable investment and you will need to decide whether you think that it will be worthwhile or not. With this system you also have the flexibility of being able to stock shops if you want to as well as selling online through any sites that you please, including one of your own.
In order to make money you will obviously need to sell the t-shirts for more money than they cost  to make. But you also need to allow for other costs as well. You will need to consider the time that it takes to design them and to set up the sales pages, any marketing that you may need to do and the tax that you will need to pay on the profits that you make. It is important to make sure that you have a market before you make a big batch of shirts and it could be wise to start by doing a made to order system so that there is no waste and test the market and then you could decide whether you will want to have a batch made up. You will have a better idea of what sizes you may need and how many as well, which should help you to ensure that you sell all of the t-shirts that you have made.