Your Mac comes with an app called Preview that you can use to change the size of your photos. Open your photo in Preview and in the menu bar menu choose Tools - Adjust size. 800 to 1000 pixels wide is usually large enough for most website uses, and resolution should be set to 72 pixels/inch.
You can scale down without losing quality - that is, you can make a large image smaller without much loss, but the same is not true for making a small image larger - it will get fuzzier as it gets larger. There is much more to learn about working with images, ppi for screen vs. dpi for print, and a lot more, but that little basic tip will enable you to resize most of your photos for use in your website or socials. For example IG photos measure 1080 x 1080px @72 ppi.
Be sure you make a copy of the photo you are making smaller, in case you need to save the large size for printed uses.