To add just a little to the post by CJ Driver... if you're starting with latitude and longitude in the standard format of degrees, minutes and seconds (to however many decimals), first convert it all to degrees. Then convert to radians in the way that he (or she) has described.
The conversion is simple enough, just divide the minutes by 60, then divide the seconds by 3600, then add the lot to the degrees.
If you've started with the coordinate format on a chart, it will be in degrees and minutes (to a couple of decimal places). The conversion for this is even simpler because you only have to contend with the minutes.
You might still end up with an error of a mile or more, depending on the datum used in the chart. The formula for deriving distance, as posted by SFI145 is a catch-all that should work pretty well for the WGS-84 datum, but not be all that great with any of the myriad of other datums that are available.
In any event, you might end up with a slightly different answer to that shown on a chart. This will be due to factors such as the way the distance is rounded for chart use. Also, on whether the chart distance was calculated using the actual coordinates, or the decimalised version of the same.