What he did
Galileo found a whole bunch a stuff about Kepler's Supernova, Jupiter, Venus, Saturn, Neptune, Sunspots, moon and Milky Way and stars. He was also famous for inventing the thermometer, perfecting the telescope and for experimenting with gravity and pendulums.
What he used to see all the stuff in the solar system
He used a telescope. It was a crude refracting telescope. His first version only magnified 8x but was soon refined to the 20x magnification he used for his observations for Sidereus nuncius. It had a convex objective lens and a concave eyepiece in a long tube. The problem with his telescopes was their very narrow when you look through it, about half the width of the Moon.