“Well done!” said Holmes in an encouraging voice. “Really, Gregson, you are getting along. We shall make something of you yet.”

What does it mean?

Here, does "get along" mean get better?
"getting along" = making progress.

"make something of you" = turn you into a worthy or accomplished person (in some way that is explained by the context).

I'm assuming you understand the rest of it. If not, post back.
Yes, I understand the rest. Thanks, Mr Wordy.