Agree with what
Pommie has said. I have used these before and they work well and are easy to understand. Tell us what your project is - that is, what are you trying to do. That will help us help you.
You would want a meter to measure voltage out. Use of a 9V battery for power may or may not be what you need as that will depend on the OP amp you are using. For understanding, those three graphs are basically assuming V+= 5V and GND = 0V. Since the OP amp is not given, let's assume that you chose an
MCP601 and V+=5V and GND=0V. Again, many different OP amps could be used.
Stick with the first circuit initially. Once you understand that circuit, you will be able to understand the other two.
Look at this portion of that circuit:
View attachment 121884
That is a simple voltage divider using two resistors; the top resistor is the FSR (which is a variable resistance, in Ohns, depending upon the force applied) and the bottom resistor is a fixed resistor of some value (in Ohms). Look
here for a decent tutorial.
When you add the op amp, you are buffering the voltage out.
View attachment 121886
At this point, some recommendation for an introduction to OP amps is probably called for. Look online for a simple one. Many times you will see these circuits used with V+ and V- (e.g., +9V and -9V) rather than V+ and GND (e.g., +5V and 0V). That might confuse you at first. I suggest that, at least initially, use an OP amp with V+ and GND (there are ones [like the MCP601] specifically for that, called single supply op amps - although EE's can take the mystery out of the +/- aspects).
Looking at the graph for the voltage out for the circuit, you can see that it depends on the value of the fixed resistor (RM) and the resistance of the FSR.
You should be able to see that so well that given the value of RM and the voltage out, you should be able to calculate the force applied to the FSR.
Now, you have to reveal what you want to do with it because it is going to determine how you use it and whether you use circuit 2 or 3 or something else.
Say, for example, that you want the circuit to "trigger" when the force applied is, at least, some amount. But you do not exactly know how much - say a tap by a finger or when something is dropped on the FSR. In that case, you want a Yes/No response from the OP amp, but you may want to adjust the threshold of the Yes/No. Take a look for an example of such a circuit (first schematic, middle). Note that there is the addition of R2 (look up hysteresis or use the link in that article - but dont worry about it at this point). That is a simple application that is used to provide an input to a microcontroller to detect when some threshold force was applied to the FSR - what's more, the threshold force could be set using the potentiometer.
As for your other two questions, you have that right, sort of, kinda...get the first part of the understanding down first - for better or worse, that is what helped me.