Hello Joseph.
Low body fat is required for abdominals to be visible. Normally this requires a strict diet and a decent amount of cardiovascular exercise.
My only concern is 140lbs and you're six foot tall?! I'm guessing you're probably already rather on the thin side because of this. Soo... if you want anything other than a silly set of visible abdominals (read, some actual useful strength) then you want to do the following three correctly.
Squat,
Deadlift,
Bench,
and squat some more.
There are lots of topics and posts on all three I have mentioned. I would think that it is wise for you to avoid isolation type exercises (and machines in particular!) for the time being.
Don't forget that you need to eat a fair amount to put on any muscular mass too, and potentially put on a bit of body fat while you're getting bigger and stronger. But this can be reduced at a later stage.
I hope this helps somewhat, and I expect someone else to be along shortly with some further comments.
Need A Little Help.
