The plunging attack works like this - drop off the edge and as soon as you're in the air press attack. Don't try to hit attack as you come into contact with the enemy because that's too late. Locking on also helps for this particular plunge I find and I used to wait for him to look up as that tells you he's exactly under you and ready.
Also you're a Thief class? That's not the best class for a first playthrough. Class matters so very little - but if you're muddling through with the low HP and knife you get as a Thief it can feel harder than it should. I would try another weapon and put some points into vitality as soon as you can. Even a humble longsword will probably feel better, and it has a great varied moveset too.
Also buy a 100% physical block shield if you don't have one yet - Undead merchant sells the Heater Shield which also has relatively high fire defense and is pretty light to boot. Don't give up, it will click and when it does you'll feel awesome every time you smash a big monsters fat fucking face in!