At first, you need to learn Adobe Illustrator (for vector design) and Adobe Photoshop (for t-shirt mockup). Adobe Illustrator is okay but Adobe Photoshop is also recommended.
So, first, learn how to use Adobe Illustrator. I would suggest 'GFX Mentor' and 'Learn with Shohagh' youtube channels for learning Illustrator from the basics. You will find the Illustrator Basic learning 'playlist' on these channels. (A-Z)
After learning the basics of Illustrator go and learn T-Shirt design. There are huge resources on youtube about this. You will find everything. For the T-Shirt design, I would recommend the 'Learn with Shohagh' youtube channel only. Besides, you will find more youtube channels.
Tips: For Design Idea try to use Behance, Dribbble, Muzli as inspiration sources. If you want to use Google, then try to filter the search for recent content only.