There are several tools that will make it easier to move your ideas from your head to your computer. The best tools feel natural, are easy to use, and don't get in the way of the creative process.
UI Design
Figma (a web-based desktop application that lets you design for any screen size)
Hero Icons (a library of SVG icons)
Pexels (a library of photos)
Utilities
Digital Color Meter (used to determine the HEX value of a color in a image or portion of the screen)