Sounds like a 'student project'.
May I recommend Horrowitz & Hill. The Art of Electronics. I once owned what became a well thumbed copy of the first edition which I kindly but foolishly gave to someone else. The second edition included lots of extra stuff about microprocessors and switch mode power supplies but retained much of the nice analogue proper electronics. I do not know whether the following link is 'hooky' but it has been up for some time.
https://pearl-hifi.com/06_Lit_Archive/1 ... ronics.pdf
You will have to poke about a bit but you will find a wealth of information dealing with what you are trying to achieve. Chapter 7), Chapter 15)... Also have a look for Dual Slope Integration A/D conversion.
As others have mentioned you are going to need an instrumentation amplifier, the three op-amp one, to interface the strain gauge bridge to your A/D converter, dual slope integration... You can build one of those including auto-zero with three/four op-amps, a comparator and a CD4066.
Look at the legs on that beauty.
You drive the CD4066 with the I/O pins of your Pi whilst watching the output of the comparator. I'll try to remember what one of those looks like. Should be good for at least 14 bits assuming the Pi and your programming is up to it...
Don't forget to read the stuff about operational amplifiers, comparators and voltage references and you will have to bone up on what strain gauges are and what they actually measure. You will also need a soldering iron and some veroboard. Do not expect to poke bits of wire in an s-dec.
Nice project. Lots to learn. However...
I am currently trying to measure strain from crack growth on some plastics in our lab with strain gauges
I might be inclined to question the proposed methodology as applied to that particular application. You might translate that as I do not understand but if it is just a 'student project' perhaps the person who has set it does not really care about the application beyond the possibility that it sounds like something that might make sense.