There are many ways you can organize your gallery photos.
The photos in your galleries are displayed by default in alphanumeric order.
The only way to change the display order of the photos is to change the filenames, which you can do by selecting a photo, clicking on the "Actions" button, and then choosing "Rename File."
If you are going to be reordering a lot of photos, it may be more efficient to rename the files in sequence in your photo editing software before you upload them.
You can sort your photos into Albums, which are like folders or groups of photos within a gallery. For example, if the "Smith Wedding" is your gallery, you might have albums for "Ceremony", "Reception", etc. For a sporting event, a photographer might set up albums for each team, game, and/or day of tournament.
Learn more about using Albums and their benefits here: Using Albums