It depends what mode you're in. If you start with very easy (or easy, I can't remember) then you start with the EZ stun gun which you can just tranq the guard at the front of the bridge and then drag him to the long grass.
If you're on normal, use your tranquiliser gun and do the same but don't waste too much ammo, so a headshot will suffice.
If you're quick enough, run towards the front of the bridge and tranquilise the furthest guard (2nd of 2) and then hide and wait for the 1st of the 2 to walk across the bridge and either sneak up on him and knock him out with a choke hold, do the 3 punch/kick combo or just tranquilise shot in the head.
That's about it. I'm saying to tranquilise them as completing the game without any kills give you a special item at the end for the 2nd playthrough.
Alternatively follow tracy6697's advice and go to gamefaqs, it will pretty much say what I've said although it will be thorough and contain sections for later on in the game, because no doubt you'll get stuck a few more times.