One has to hold the bat with one or both hands and stand in front of the stump. Its called the stance.   A stance is subjective to 3 sticks (stumps) that are placed behind you. You need to stand in front of it.   The three stumps are off, middle, and leg.   One has to stand on leg stump so that one is aligned with the line of the throw(bowling) which comes from approx. 22 yards.   The purpose of a batsman is to score runs(with his batting partner) and save his wicket. A run can be scored either with the bat that one is holding, or from legs which should not be intentional act. One can also get runs if fielder(wicket-keeper) behind the stumps misses to collect the ball delivered by bowler. 
When selecting a bat one needs to watch the balance and comfort of it. It should be an extension of our hand.
What to look for
1-    Not too light, or not too heavy.
2-   Not too long, or not too short.
3-   Not too broad, or not too slim.
4-   Not very thick, or not very thin.
5-   Feel comfortable by picking a bat with gloves/inners.
6-   Never get tempted by brand or particular batsman’s choice. Its customized for each one, and you can do that.  

Purpose of bat:

a)   The purpose of the bat is to meet the ball at any length, line, flight, and height, either to score runs in guided direction, or to defend/block from balls hitting the stumps. Hence it should not be a burden, but an extension of an arm.

b)   to score runs in guided direction, or to defend /block balls from hitting stumps/pad).

c)    Carry the bat while running. Hopefully for longer period of time.

Preparation when selecting.  
a)   When buying a bat, always wear gloves with inners inside it. This is the attire of a bat, which is held for hours, days, session, overs.
b)   A bat is carried along while taking a run, and longer one bats, more running is done with bat in hand. Hands get tired, and sometimes after a long stay at the crease, a batsman can skew his bat, resulting in losing his wicket.
c)    We increase and reduce weight regularly and successfully, but the size and weight of bat remains same(marginal loss of wood occurs, if it is daily). Nevertheless as we get older we need to strengthen our hand to carry the same bat.
d)   Advisable to buy more than one bat, light and substantial.