For example, if I wanted to make a T-shirt of a character for the Arrow Tv series, but I drew it myself/designed it myself. Is it legal for me to sell products (T-shirts) of it?
You can make a derivative work if:
- the original is not under copyright,
- you are the copyright owner,
- you hold a licence from the copyright owner that says you can, or
- your usage is fair use or fair dealing as applicable.
For your proposal, the image is copyright, you don’t own it, you don’t have a licence and what you propose is neither fair use nor fair dealing. You can’t do it and you can be sued if you do.