Like I stated several weeks ago, you need to build tension from page 1, the first paragraph, if you can. Then, by the end of the first page, the story line should be so engrossing that the reader can't put it down.
Like I said several weeks ago, have the protagonist in some kind of trouble, or as Dean Koontz says "terrible trouble," right off. Give the protagonist a challenge. Then, as the story progresses, as the protagonist tries to solve his/her terrible trouble, he/she gets deeper and deeper into more trouble until at the very end, the climax if you will, the problems are all solved.