//at the top of ur script
new CheckPoint[MAX_PLAYERS];

enum 
{
cp1,
cp2,
cp3,
cp4,
//and so on...
}

//first a stock function to create the cp with ur wanted id's
stock SetPlayerCheckpointForAll(ID, playerid, Float:x, Float:y, Float:z, Float:size) // Creating a new function that includes an ID parameter
{
    CheckPoint[playerid] = ID; 
    SetPlayerCheckpoint(playerid, x, y, z, size);
}

//now create the cp:
SetCheckpointForAll(cp1,681.3374,-463.3429,22.5705,2.0);//cp1

//now under the callback OnPlayerEnterCheckpoint u can set the action that shall be executed as soon as a player enters a specific cp.

public OnPlayerEnterCheckpoint(playerid)
{

//checkpointselect
switch(CheckPoint[playerid])
{

case cp1: 
{
//function for the first cp.
e.g. u can disable the first cp with DisableCheckpointForAll();
and create the second one

SetCheckpointForAll(cp2,681.3374,-463.3429,22.5705,2.0);//cp2

}
case cp2: 
{
//function for cp 2
}
case cp3: 
{
//for cp 3
}

public OnPlayerSpawn(playerid)
{
    SetPlayerCheckpoint(playerid,1515.3851318359,-1464.0179443359,9.5,5);
    return 1;
}

public OnPlayerEnterCheckpoint(playerid)
{
    SetPlayerPos(playerid,3053.7912597656,3649.2023925781,3.6554095745087);
    return 1;
}

public OnPlayerEnterCheckpoint(playerid)
{
    if(IsPlayerInAnyVehicle(playerid)) SetVehiclePos(GetPlayerVehicleID(playerid), 3053.791, 3649.202, 3.655);
    else SetPlayerPos(playerid, 3053.791, 3649.202, 3.655);
    return 1;
}