I would recommend Illustrator as it is a vector program that has also been designed for UI design. Photoshop on the other hand is a photo editing software, so it does lack some features when it comes to UI design because the program Photoshop was designed for photo editing, not for UI design. Though as you are just starting UI design, I recommend you start with Roblox Studio and get to know some basics of UI design before you begin UI design right away.
Here’s some tutorials on UI design for starters:
Here’s also a DevForum thread that has a bunch of Game Development Resources listed on it (this thread includes UI design resources plus other game development field(s) resources):