Just adding some info to stevek666's info: Most baseball game lists the player's prior year's stats, so I think it'd be relevant if I give you numbers based on a whole single season.
Hitter Average:
Pretty much what steve said. Walks do not count for your average.
For example, in one game, let's say the batter gets to come up to the plate 4 times. He gets 1 hit (a single, double, triple, or a home run), strikes out twice, and walks once (by getting 4 balls). In that instance, the batter batted .333, or 1/3, and the walk is not calculated - the walk would go go into your OBP (on base percentage), but I dunno if earlier baseball games calculated that).
In real baseball, someone who hits at least .280 is respectable, .300 would be a star-level batter, and batting .350 for the year would be friggin incredible.
Home Run
Once again, in real life, someone who hits 25+ Home Runs a year would probably be good enough to bat in the "clean-up" of the lineup (3rd, 4th, 5th in batting order - the ones who are mainly responsible for getting your runs for your team). 30+ Homers & you're likely a star player, and anything above 40, you're one of the marquee players in the league.
ERA
Lower the better. The basic formula is how much the pitcher on average would give up pitching 9 innings. So if he has an ERA of 3.5, that would give up an average of 3.5 runs per 9 innings. Calculating this can get nitty-gritty (like runs scored on errors don't count, and if any runners are left on base if the pitcher gets changed and the runner scores, then the run is charged to the former pitcher), but on the whole, just know that an era of anything in the 4.xx would be an average Joe pitcher in real life, someone in 3.xx would probably be a star, and 2.xx would be a major star, and you may only a single pitcher or two every 10 years who can get their ERA's in the 1.xx's.