Considering only flexural stress of the steel beam, you can derive an equation for uniform load w from these equations:

   M = Fb S
   M = w L^2 / 8

solving for w,

   w = 8 Fb S / L^2
   where Fb is the allowable bending stress

There isn't one single simple formula for obtaining the maximum uniform load a beam can support as it depends on the support conditions.  In addition, flexural stress may not be the controlling factor since for long spans, beam deflection usually controls.

Invest in a relatively inexpensive steel beam program called RAMSBeam from http://www.ramint.com/products/r_sbeam.jsp

You should be looking for a professional.  There are more things to do than simply sticking numbers in an equation.  You have to know which numbers, where they go, and which equations to check.  

I second what UcfSE stated. There are many considerations beyond just bending stress level for instance acceptable deflection, lateral bracing, shear, connections, etc.

I agree with UcfSE and jike.  Deflection would be a huge concern for me with a W8 spanning 24 feet, even if braced and properly connected.