How do I make some reference entries bold in bibtex?
It depends on the bibliography style you're using. But you could use the common commands to choose bold typeface:
For instance, with this file
and that simple example
you would get:
The bold typeface has been added to the italic shape of the plain style. It's just an example, I would not combine font properties this way.
A more consistent way would be to modify the bibliography style according to your needs. Or use biblatex.