I'm looking for a good LEGO Modeling Program
The most commonly used one is Lego Digital Designer (often called LDD).  This has the advantage of being free and quite easy to use, including automatically snapping connections together.  However, it is only available for Windows and Mac.  Also, LEGO has discontinued support for it.

There is a system of programs based on LDraw.  These programs tend to be harder to use, but much more flexible.  Some of them, like LeoCAD, are fairly user-friendly.

A recent arrival is Stud.io.  It's in open beta but seems quite user-friendly.  It also has the advantage that it integrates with BrickLink.  It will tell you how much a certain part (or your whole model) will typically cost on BrickLink, and if a certain part is unavailable in a certain color.

One caveat is that none of the above solutions has all available parts in it.  Most common parts are available in all three, but some of the less common or older parts are not.  Some parts -- especially the very new, or very specialized parts -- are not available in any of them.