there is many positions in the infield like the following. First basemen is the one that is most used players in a game. For a first basemen strength and size (particularly height) are important a strong throwing arm is not. A first baseman needs to be able to concentrate, as he will potentially be involved in nearly every play. Next there is Second basemen. For them size and stature are of little importance also Speed, quickness and good fielding ability are good for them to have. A second baseman needs to know what to do when there are runners on base. Next there is the shortstop they usually have more ground to cover. The shortstop must be fast, quick, agile and have a strong throwing arm. The shortstop will probably field more ground balls, in more off-balance positions, than any other player. Like the second baseman, the shortstop needs to be to think ahead. The third baseman should be able to charge the ball on bunts and slow grounders, and field the ball barehanded, and be able to move side to side quickly on balls hit hard down the line or to his left in the hole between third and short. Because the third baseman has the longest throw of all the infielders, he should have a good arm. (http://www.momsteam.com/sports/baseball/ the-sport/what-are-the-different- positions-in-baseball#ixzz4ikmUE2TS).
The main quality you’ll look for in a first baseman is the ability to catch the ball. They should be agile and not be afraid to get in front of the ball. This player is not afraid to try anything to catch the baseball. A taller player is typically ideal, as they’ll present a larger target to the fielders and be able to stretch to catch inaccurate throws. Every second baseman should be quick on his feet. You’ll want to find a player who can field the ball well by getting in front of it, and who shows some capacity to backhand a ground ball. This player doesn’t need to have the strongest arm, as most of his throws will just be to first base or second base, both of which will be less than 30 feet away if the player is properly positioned. This player shouldn’t be afraid of the ball. The shortstop should be one of of your best defensive players. The shortstop should show great range and the ability to field sharply hit baseballs. Choose someone who has an above-average arm, as many of their throws will be a great distance. The third baseman has the longest throw to first base, choose someone who has a good arm and player capable of making strong, accurate throws that will beat runners to the bag. Third base is called the ‘hot corner’ for a reason. Third basemen can’t be afraid to get in front of the ball, and they have to be quick because they’re responsible for fielding bunts most of the time.
(http://protips.dickssportinggoods.com/sports-and-activities/baseball/defensive-lineup-strategies-for-youth-baseball)