Canva is an online, browser-based software that let you design items for printing on sublimation, stickers, laser and more. It has a free version and a paid version.
We have a number of templates created at the right size for various projects that you can use.
Sublimation Templates in Canva
You should start with fabric scraps, coasters, then mousepads. The tumbler is the hardest, so save that for when you feel you have the skills down.