aaaah et

....
ex top 10 clan player right here lol ... in australia though ... pretty awesome community here...
but anyway, as an engineer you have a number of responsibilities, you construct switches,guns,barriers etc by approaching the "piles of boxes with coloured flags above them" ... as allies, go to the piles of crates/boxes (usually about waist heigh) with the blue flags, as axis head for the red flags.
As well as this, an engineer plants mines (keyboard shortcut 8 i think :s .. quit about a year ago) which can damage both tanks and personnel.
Your C4 explosive can be armed by first selecting it as your weapon, clicking fire (which will "drop" it) then arm it using your "construction pliers" by holding down fire when your near it and crosshair over it..
basically, use the pliers, look for your corresponding flag, and learn where to plant game-saving c4's through experience depending on the map. On some maps (eg: oasis, battery) the engineer must plant c4 at a specific target in order for their team to win ...
its a top game if you get into it, good luck and hit back here for any more et questions :d
edit: just to reconfirm it, once youve approached your "boxes/crates", select your construction pliers (which will be part of your weapon arsenal) and hold fire while the progress bar moves across ... beware this makes you an easy target so depending on the situation, id exploit the hitboxes by proning near the crates and constructing ...
god if thats not clear soz lol ... again though just reply if theres any queries