First understand the information hierarchy.
A loadable family defines types that are used to create multiple instances. So for example:
A rectangular table (family) defines two types (1800x1100mm table and 1500x1500mm table). So, maybe you created three 1800x1100 tables in your project and positioned them on three different places. Revit defined three instances of the same type. However, one of those three can have slightly different properties (for example, that could be a different color of the varnish...). This is why Revit differentiates type properties from instance properties. When you create any family parameter, you can choose whether it will be a type or an instance property.
To constrain the geometry of the family with newly created parameters, you must label dimensions. Those dimensions are actually distances from the reference planes and the geometry should be locked to those planes.
To start learning about the family creation, read this help topic. Also, feel free to explore Youtube. There are many video tutorials. Since what you need right now is a very basic Revit lession, any good rated video you find there will not give you bad information.
Andrej Ilić
phonetical: ændreɪ ilich
MSc Arch
Autodesk Expert Elite Alumni
