To be able to manipulate your object, you will always need the Selection tools.
Selection mode allows you to select the specific area that you need to manipulate in order to achieve your desired needs.
In SelfCAD, the selection tools are found in the right-hand side of the workspace as shown below:
To be able to select your object, you can click on it or click on the box on the scene section and vice versa.
Additionally, you can also select your object (s) easily using the Marque Selection tool. This is done by dragging and dropping around the object or objects that you need to select and leave out the rest. And to deselect the objects, you will repeat the same process as above.
Check Gif below to learn.
You can also select all the objects using the command CTRL+ A.
To select specific sections of your model, you can use the following tools.
Inverse Selection Tool (R+I) 
Allows you to invert your selection, both region, and object.
Polygon selection Tool (R+P) 
It’s used for either selecting the polygons of the model or for selecting the faces of the model.
Check this Gif to learn how to achieve this.
Edge Selection Tool (R+E) 
It’s used for selecting the edges of the Model.
Cube Selection Tool (R+C) 
Used for selecting the object in a cube-wise manner as shown below.
That's how you can be able to select objects in SelfCAD!