AM or FM?
What kind of setting? Wooden or steel-framed buildings? Hilltop or valley? City or small town? How far away is the station you are wanting to receive?
In general, what you'll do is set up a conventional FM antenna with a 75-ohm line. Connect the center lead to where the built-in antenna enters your radio (leave it folded up short) and the cable shield to circuit ground (probably the shield of the earphone cable).
Or you might try simply adding length to the existing antenna. It's presumably a 1/4-wave vertical (30 inches long). Try tripling this to 3/4 wave (90 inches) by adding a piece of wire clipped to it and extending as near as possible to straight up.