First go to the gunner's position, usually by hitting "2". Then zoom in with the "z". Adjust the zoom by holding "[" or "]" to go in or out.
The gunsight has a kind of ladder, showing how high you need to pitch the guns to kit at certain ranges. The pitch is higher for High Explosive than for Armor Piercing; you switch between ammo by hitting backspace. The numbers by the ladder lines refer to ranges -- they drop zeros to save space on the sight, but yuo can figure it out.
Practice offline or in TA, adn you should be able to get used to it OK.