After you read the second part of the introduction, you will no longer want to play a psyker
Besides, at low levels psykers are HORRIBLE. I've played one. The same session I was *finally* able to cast the cool abilities (rank 3) I botched a power and became possessed by a daemon and killed the whole party, after which I was hunted and slaughtered by a group of Grey Knights.
Oh yea, did I mention psykers kill the party often? I would wager that in my tabletop group, 70% of our fate has been spent surviving psyker mishaps.
Until Rank 3, they are way overrated, sort of like wizards until they get 3rd level spells.
(edit: fixed the first sentence of the second intro.)