Your 75 AT Gun also fires HE and AP, as signified by the lack of underlines either above or below the 75 mm gun size.
HE will be one of your best options against infantry, better than HEAT because you can fire at infantry in the open as well.
There are two steps to obtaining a result with any Ordnance (Gun or Vehicle) weapon. First, you need to see if you hit. Second, if you hit, you need to check what damage you achieved.
To see if you hit your target, you roll a TH DR. First, cross reference the distance to the target with the target type. You'll probably be using the Infantry Target Type, which has a Basic TH of 8 for a range of up to 6 hexes from your Gun.
Then, you'll roll the dice and add any of the DRM on the C5 and C6 tables to the DR to produce a Final DR. This includes Case K (Concealed Target, +2) and Case Q (TEM, +2).
Let's say they were the only DRM's that applied. To have hit your target, your Final DR must be less than equal to the Basic TH number. So if the Basic TH was 8, and your DRM were +4, you'd have needed to have rolled a DR of 4 or less to hit.
Okay. That's stage one done. Now what did you achieve? Was it all in vain?
Now you roll a second time, but this time on the IFT to find the damage result. Use the IFT column that corresponds to the mm size of the Gun you were firing, or the next column down if your gun is an inbetween size (like your gun). So a 75mm will be resolved on the 12 FP column.
Go ahead and roll. Note that TEM does not apply to the resolution. That's because when you fire Infantry Target Type, the TEM has already been accounted for in modifying the chances you hit the guys in the first place.
And that's pretty much it, for a basic run through. There are other odd things that you might run in to, like Critical Hits and the like, but this is probably good for most cases.
One final thing to consider... you could also fire using the Area Target Type. There are two main differences. First, TEM doesn't apply as a DRM to the To Hit roll. So using the example from above, at a range of 6 the Basic TH for Area Target Type would be 7. Your Final DR would only have the +2 DRM for firing against a concealed target. So a 5 or less would hit; slightly better chance than with the Infantry Target Type.
The downside is that in exchange for a better chance to hit, your FP is reduced on the IFT resolution. Instead of the 12FP column, you'd halve it to the 6 FP column. Plus, TEM would act as a modifier on the roll.
Hope this at least helps get you going.