Instrumental magnitude to "real" magnitude - Photometry with not enough data?
The easiest method of photometry is by comparing the measured instrumental intensity of the unknown target with one or more other known targets in the same field and then converting the difference into magnitudes.  This is called differential photometry, and is easier because you don't have to worry about things like atmospheric extinction.  The more comparison stars you use, the better your result.  There are tools that will automatically do this analysis and calculate your error bars.  I use MaximDL.

Visit http://www.aavso.org and use the Variable Star Plotter (VSP) tool to generate a chart for your star of interest.  This will give known magnitudes of comparison stars in the same field.  You can use those to determine the magnitude of your target.  While you're there, check out some of the FAQs and documentation on variable star observing, and submit your results to the online database.