There are a variety of ways to create NURBS models. This is a summary of how you create a top-level, parent NURBS object:
You can create a NURBS curve on the Shape panel of the Create panel.
You can create a NURBS surface on the Geometry panel of the Create panel. When you use this technique, the NURBS surface is initially a flat rectangle. You can alter it using the Modify panel.
You can turn a standard geometry primitive into a NURBS object.
You can turn a torus knot into a NURBS object.
You can turn a prism extended primitive into a NURBS object.
You can turn a spline object (Bezier spline) into a NURBS object.
You can turn a patch grid object (Bezier patch) into a NURBS object.
You can turn a loft object into a NURBS object.
To turn objects other than NURBS curves and surfaces into NURBS objects, use the Modify panel. Right-click the object's name in the stack display (see Modifier Stack) and choose Convert To: NURBS.
In viewports, the quad menu also lets you convert objects to NURBS. Select and then right-click the object, and in the Transform (lower-right) quadrant, choose Convert To: > Convert to NURBS.