This isn't much of an answer, but more of an explanation and suggestion. Because the game of poker is so dynamic, there is no "real" answer, thus I continue: The percentage of pocket cards you should play vary depending on the amount of players there are at the table, and if you are playing a tournament or a ring game. Good players play very tight at the beginng of tournaments, slowly loosening their acceptable criteria as the number of players go down, and their bankrolls go up. Another factor thrown into this is the "style" of the table. The style is basically the attitude of the table, be it aggresive, conservative, tight, or aggressive. Poker is also a thinking-man's game. When you gain the respect of the table, you can play sloppier cards at certain times. LOTS of factors, even yet! The point I make is: Educate yourself on the theory of poker, the education you get will dictate which cards you choose to play based on the situation you are in. If you absolutely need a precentage, there is no totally agreed upon number, but most pros state that at a full (10 seat) table of amateur to intermediate players (usually micro to 5/10), you should on average be in about 10-15% of all hands; which equates to playing roughly the top 34% of all possible starting hands. Most books will teach you the basics, but the best authors to keep an eye out for are David Sklansky, Mason Malmuth, or Doyle Brunson. If you have a favorite player, feel free to pick up their book or video. I hope this helps!

what percentage should you call?! interesting question, and as the first answer says - it depends on so many factors. if you don't want to lose money too quickly, i would 'play only about one in 5 hands. only the best cards, very high pairs or high suited conectors (ie AA, KK, QQ, or AK, KQ, QJ - same suit). To be double sure, play them only when in late position - ie on a ten person table, when you are in position 7-10. 'calling' a hand is entirely different, and certainly is not answerable in a few sentences, or even a few paragraphs (maybe a book would do it). Learn as much as you can, then see how often you actually do call (pre flop, post flop, on the turn, on the river), if you play aggressively, you probably won't call that often at all, you'll be betting, raising, or re-raising.

[edited - posted same thing twice!]