1) Create a t-shirt:-It is better if you design your t-shirt model first in a paper then to image creator. First of all you have to open a blank page and create a t-shirt. That is first create a rectangle then edit its top and from the two lines create the hands of the t-shirt. Just create the shirt like you draw on the paper.
2) Give a background-:Some background colours like black, gray, yellow, red makes attractive. Even whitld background is also good for the t-shirt. Mixing colours will also good for background but you have to be careful that your background colour is attracting to the design seekers. Don't give much darker colours it will not be much attractive light colour are found to be the best.
3) Make design-:Now on the just part of the t-shirt you have to add some attracting text or some clip arts. Some texts like 'Leave Me Alone I'm free for lonely' are good to attract the youths or some images or clip arts will be also attractive. Make sure that you have to create it well because it is the main part of your work.
4) Finishing touches-:Just edit all the unwanted things from your design. After creating the design you need to be sure that the design is attrative. It is necessary thing that don't copy text or clip arts from others work because it will lacks the trendeness of the shirt.
5) Upload it to online community-:I already mentioned about two communities that is 99design and freelancer, this two are an effective place for promoting your design. In both communities there are many t-shirt companies are waiting for new design so just upload it to any one of the companies. Generally a t-shirt design may cost about 120$ so with t-shirt designing you will get paid from companies.
This is the way to create t-shirt design. One thing you have to be careful is that don't give your design to multiple companies because they may reject as your already given to another companies and also sometimes your new design also will not be accepted so be careful about that also. All the best.