If you're looking for a tricep isolation exercise then my vote is for skull crushers. I use an easy curl bar as I can then get my hand angles right to really hit the middle head for maximum power but it works the whole group and I get a huge pump.
But I'm with others in agreeing go compound and not isolation to start with. Do some big bench work and your triceps will come and you'll get the added bonus of chest and shoulders. Focus on bench, squats, deads as the three core lifts in your programme across a three day split and add in assistance exercises to each. Or, just do all three on an alternate day programme like Markos' starting strength. Whatever, just make sure you do these lifts as the core of your programme.
I remember a great comic frame which had this immense unit talking to this ripped little guy. The little guy was saying stuff like: "I do leg extensions, db lunges, single leg cable hamstring curls, calf raises seated and standing and still I can't bulk, what do you do?" And the big guy just says: "Squats." You get the idea...
Cheers,
Mike