how do I get my photos on my computer to be able to be shared online with websites? (uploading from computer)

I'm guessing you are trying to 'drag' the pictures to an upload link? That may be why your having a problem, your best to export the pictures to a folder on your computer. Select the pictures you want to share, then, in the Photos menu: File> export and send them to that folder (you'll notice under File there is an option to share, this does send straight to some sites if you have accounts with them).

Upload the pics to your chosen site from the copies exported to the folder. This is to do it on a computer, I'm sorry I don't know of a way to do it from an iPhone or iPad as it doesn't have a way of putting them in a folder elsewhere.

If the website has a "Choose Photo" section then click on it and in the browser's selection window look for the Media section at the bottom of the sidebar.

Click on Photos under it and then on the right on the Photos library. I find it easier to find what I'm trying to upload by putting them in an album created specifically for uploading at or near the top of the library's sidebar.

Do you mean Instagram, Facebook etc? Your whole library or some individual pictures?