How can a beginner decide which photos are worth keeping after a shoot?

The first question you need to answer here is "What is a good picture?", unfortunately this is subjective, and also context-dependent.

Consider, if you will, a blurry photo (for example, due to motion blur, lens shake or just plain out of focus). Is it good or bad? From what I've said so far, who can tell?

If it's a shot from a wedding, from the paid photographer, and supposed to be of "the kiss", then no, it's probably not a good photo (and you may not even be able to bring yourself to look at it).

However, what if it was a photo taken of you with a much-loved relative just before they unfortunately died? Surely, then it would a treasured image?

Of course, the reality is usually nowhere near as extreme as the above examples!

So what you need to bear in mind is what you were trying to capture in the instant you hit the button - did you get this? If so, then it's a good photo. If not, then maybe you captured something else, unexpectedly, and it's still a good photo! If none of the above then perhaps it's bad. But personally I never delete any image I've taken (and I use Canon 5D series bodies, so they are large images!), what I do is archive the raw data, then select the ones I want to edit, then select the ones I want to show. Each selection uses the criteria given at the start of this paragraph.

Lots of people will bang on about balanced histograms, focus points, rule of thirds, depth of field and other such stuff. But many of the best images ever taken have ignored at least one of these points. Remember: The rules are there to make you think before you break them.