I'm making several pages that have several images on each one. The resolution is set to 72 pixels and I don't know how to change that and make the resolution higher. I'm trying to make these pages very detailed with high resolution
You are limited by the overal pixel dimensions. Go to Image/Image size and turn on Resampling.
This will adjust your size as you increase your resolution.
The canvas is set at 72 pixels/ inch. Is there a way to make that higher?
Adding to what Kevin has told you, when working with any sort of montage, or composite, dpi is essentially meaningless. To create a high resolution end product, you have to start with high res originals. Using resample to increase the pixel size will give you more pixels, but they will lose detail.
How many images will you have on each page? What size (in pixels) are those images?
Incidentally, while you can use Photoshop for page generation it is not the best tool for the job. Adobe's inDesign is the optimum tool, but if you don't have that, you might have Publisher if you have the MS Office suite installed.
In order to get the images I'm using I have to screenshot them with the command shift 4 on my mac. I'm creating a field guide to dental anatomy of fossils
It is unlikely that screen shots will be of sufficient quality for printing. No magic can fix this, you need better originals. (Also, be aware that whatever you find on the web is copyright, including picture or drawings of fossils).
Screen shots are not going to be enough pixels/resolution to give you any detailed quality.