If you are looking for a parametric beam that you can adjust then you need to build one out of one of the family templates. I would suggest starting with the generic line based template because the structural components built into Revit act the way structural engineers want them to. In other words in a structural plan view they do not touch the walls they are supported on. If you build your beam using the generic line based template you can adjust the behaviour of the model any way you want. The family templates that are wall, floor, ceiling based have built in parameters that make them a little less flexible. The tutorials on families in Revit are very good, I would start with those. Also, you may want to get Paul Aubin's book Mastering Autodesk Revit Building. Chapter 9 is on families and is one of the longest chapters in the book. The web site DGCAD also has very good Revit tutorials. RevitCity has some great links to Revit blogs where you can find a lot of great tutorials also.