Gauge (g) is the thickness of the needle (the pointy metal part of it). Bigger number = smaller needle (weirdly...) Use thicker needles (smaller gauge like 18-21g) for pulling juice out of a vial, much easier than a thinner needle. Thinner needles (22g+) are used for injecting as they pierce the skin easier and less painfully + don't leave scars.
I personally use 21g to pull gear out of a vial and 23g to inject. Though its ok to use 21s to inject too if you run out of thinner needles...
Cc = ml
3/4 = the size of the needle in inches. This may be too small to reach gluteus muscles in the ass or quads, depending on body fat %. Anything under 10% you should be fine with 3/4 inch. Anything over I'd suggest at least an inch, maybe 1.5...