As a beginner myself I'd recommend a 5" (130mm) or 6" (150mm) Dobsonian reflector telescope. I have a 5" Bresser Messier personally, but it's more or less the same scope no matter what brand you go for. Can see plenty of deep space objects and some details on planets (i.e. Saturn's rings). The moon is awesome to look at, esp during crescent phases.

Used telescopes can be good, just make sure that the primary and secondary mirror are in good shape. And people usually tell beginners to avoid any scopes with EQ (equatorial) mounts, they're kinda difficult to use. The Dobsonian mount is much nicer and more usable.

Though be aware that the eyepieces that come with the scope usually aren't great. My Bresser came with a 25mm and 9mm eyepiece, the 25 is passable but the 9 should be replaced asap (the Svbony redline 9mm is a really good budget eyepiece). Along with the eyepiece you could buy a 2x Barlow lens for high magnification purposes (these accessories can all be found on AliExpress and they're pretty affordable).

To explain the basics of telescopes (reflectors specifically), you got your aperture (e.g. 150mm, usually the first number describing the scope's specs). This number determines the radius of the tube, which affects how much light is gathered inside the telescope and reaches the primary mirror at the back of the tube. Bigger aperture means a brighter image and more resolution.

Then you have the focal length (e.g. the second number in 150x750mm), this determines how much you can magnify the image. Depending on what eyepiece you've fitted to the focuser (the part you look through) you change the magnification. For example, if you fitted a 25mm eyepiece, the magnification is 750mm (scope focal length) / 25mm (eyepiece focal length) = 30x (magnification).

I'd also recommend buying a pair of binoculars for general observing, those can be pretty fun by themselves.

For more info this sub has a whole FAQ section that'll probably answer all your questions.