Grapple is quite complicated so first off lets start off with this. This is what a grappled creature can or cannot do when grappled.
A grappled creature is restrained by a creature, trap, or effect. Grappled creatures cannot move and take a –4 penalty to Dexterity. A grappled creature takes a –2 penalty on all attack rolls and combat maneuver checks, except those made to grapple or escape a grapple. In addition, grappled creatures can take no action that requires two hands to perform. A grappled character who attempts to cast a spell or use a spell-like ability must make a concentration check (DC 10 + grappler's CMB + spell level), or lose the spell. Grappled creatures cannot make attacks of opportunity.
A grappled creature cannot use Stealth to hide from the creature grappling it, even if a special ability, such as hide in plain sight, would normally allow it to do so. If a grappled creature becomes invisible, through a spell or other ability, it gains a +2 circumstance bonus on its CMD to avoid being grappled, but receives no other benefit.
Now to perform a grapple you use the following rules-
As a standard action, you can attempt to grapple a foe, hindering his combat options. If you do not have Improved Grapple, grab, or a similar ability, attempting to grapple a foe provokes an attack of opportunity from the target of your maneuver. Humanoid creatures without two free hands attempting to grapple a foe take a –4 penalty on the combat maneuver roll. If successful, both you and the target gain the grappled condition. If you successfully grapple a creature that is not adjacent to you, move that creature to an adjacent open space (if no space is available, your grapple fails). Although both creatures have the grappled condition, you can, as the creature that initiated the grapple, release the grapple as a free action, removing the condition from both you and the target. If you do not release the grapple, you must continue to make a check each round, as a standard action, to maintain the hold. If your target does not break the grapple, you get a +5 circumstance bonus on grapple checks made against the same target in subsequent rounds. Once you are grappling an opponent, a successful check allows you to continue grappling the foe, and also allows you to perform one of the following actions (as part of the standard action spent to maintain the grapple).
Move
You can move both yourself and your target up to half your speed. At the end of your movement, you can place your target in any square adjacent to you. If you attempt to place your foe in a hazardous location, such as in a wall of fire or over a pit, the target receives a free attempt to break your grapple with a +4 bonus.
Damage
You can inflict damage to your target equal to your unarmed strike, a natural attack, or an attack made with armor spikes or a light or one-handed weapon. This damage can be either lethal or nonlethal.
Pin
You can give your opponent the pinned condition (see Conditions). Despite pinning your opponent, you still only have the grappled condition, but you lose your Dexterity bonus to AC.
Tie Up
If you have your target pinned, otherwise restrained, or unconscious, you can use rope to tie him up. This works like a pin effect, but the DC to escape the bonds is equal to 20 + your Combat Maneuver Bonus (instead of your CMD). The ropes do not need to make a check every round to maintain the pin. If you are grappling the target, you can attempt to tie him up in ropes, but doing so requires a combat maneuver check at a –10 penalty. If the DC to escape from these bindings is higher than 20 + the target's CMB, the target cannot escape from the bonds, even with a natural 20 on the check.
If You Are Grappled
If you are grappled, you can attempt to break the grapple as a standard action by making a combat maneuver check (DC equal to your opponent's CMD; this does not provoke an attack of opportunity) or Escape Artist check (with a DC equal to your opponent's CMD). If you succeed, you break the grapple and can act normally. Alternatively, if you succeed, you can become the grappler, grappling the other creature (meaning that the other creature cannot freely release the grapple without making a combat maneuver check, while you can). Instead of attempting to break or reverse the grapple, you can take any action that doesn’t require two hands to perform, such as cast a spell or make an attack or full attack with a light or one-handed weapon against any creature within your reach, including the creature that is grappling you. See the grappled condition for additional details. If you are pinned, your actions are very limited. See the pinned condition in Conditions for additional details.
Multiple Creatures
Multiple creatures can attempt to grapple one target. The creature that first initiates the grapple is the only one that makes a check, with a +2 bonus for each creature that assists in the grapple (using the Aid Another action). Multiple creatures can also assist another creature in breaking free from a grapple, with each creature that assists (using the Aid Another action) granting a +2 bonus on the grappled creature's combat maneuver check.
Another useful thing here is that we have this nice flow chart that will show you what to do.
http://www.d20pfsrd.com/images/grapple_flow_chart-01.png?attredirects=0
