Hi Kinder and welcome.

To find out what size you can design to, it is probably a good idea to get some quotes from the printers printing your shirts based on A4 size or smaller. A3 size is too big for a tshirt IMO. Unless the people you are designing for are of a generous body size.

You don't need a t-shirt template. Just design to actual size - print it out on your printer at home and cut it out to see if it is the right size for a shirt. You could draw a rough template if you need the printer/client to know exactly where the design is going on the shirt. Check with the printer beforehand if they will print in non-standard parts of the shirt. Many printers will only print on the middle front or middle back.

Find out how many colours you can design in. Many printers charge per colour.

Find out what kinds of files the printer accepts. Ask them for a specification sheet and work towards this.

^^^ what she said.

You'll need to know which printer it is going to so that you can set up your files properly.

Screen printers typically do one color at a time. Some do not do photo-type screening, and others can do photos. So it's best to check out what you can do, before even starting your designs.

Thanks for the answers