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There are plenty of resources on the web for reading about how to calculate stresses in a system of trusses. West Point has an activity I'd like to try in a physics class. Below is an example I have started to illustrate.
In the truss above, all horizontal members have length 
and vertical members have length 
The system is statically determinate if 
where 
is the number of joints, 
is the number of members, and 
is the number of reaction forces. There are 7 joints, 11 members, and the pin at 
has 2 reaction forces (vertical and horizontal), and the roller at 
has 1 reaction force (vertical). So the system is determinate.
Here we have added the reaction forces to our diagram:
To find the tensions in the members:
- Ensure net vertical force is zero. Since
, we know that 
. - Ensure net moment is zero. The moment about is
, so 
. - Ensure net horizontal force is zero. So
. - Now we look at the forces at the joints, choosing joints in the order we can solve them. We treat tensions as positive forces. has two reaction forces, and two unknown tensions
and 
and so is a good starting point. We could draw a FBD, but since horizontal forces must net to zero, 
And since vertical forces net to zero, 
- Now we have enough information to look at . Draw the FBD. Net the vertical forces:
