You can generally split any two cards of the same value.
For example if you are dealt two 7s, these can be split. To do so, place a
bet of exactly the same amount as the original bet next to where your original
bet is located on the table.
If you are holding your cards, you should place them face up, side by side in
front of you with some room in between prior to placing the new bet.
In effect you are now playing two hands.
When it comes to playing these hands, the players right hand is played first
until the player stands on that hand, busts or passes. This will then be
repeated for the left hand. Any two ten value cards can be split for instance
a 10 and queen, however players rarely split 10 cards as the total hand value
is 20 and has a good chance of winning.
Two aces can, and should be split, but you will only be given one more card
for each hand.
Normally the casino will also allow you to re-split after the original split
if you get another pair. Some casinos have limits on re-splitting aces and if
the player is unsure of the casino house rules he should ask the dealer.