This video shows an example of how to compute the reaction forces and internal (axial) forces in a determinate truss.
The unknown reaction forces are solved using the equations of equilibrium (i.e., statics). Then the method of joints or the method of sections are used to solve for the axial forces in each element - both methods work similarly by considering equilibrium of some component of the truss.
Using a consistent sign convention, we can differentiate between elements in tension and compression, which will later aid us in design of trusses.