Command:
if (strcmp(cmdtext, "/cd", true, 3) == 0 && cmdtext[3] == ' ')
{
	if (!strlen(cmdtext[4])) {
		SendClientMessage(playerid,COLOR_YELLOW,"Usage: /cd [time]");
	} else if (strval(cmdtext[4]) > 10 || strval(cmdtext[4]) < 1 || strlen(cmdtext[4]) > 2) {
		SendClientMessage(playerid,COLOR_YELLOW,"Max time: 10 seconds");
	} else if (cdcount > 0) {
		SendClientMessage(playerid,COLOR_YELLOW,"Another countdown is already running!");
	} else if (!IsNumeric(cmdtext[4])) {
		SendClientMessage(playerid,COLOR_YELLOW,"Numeric entries only!");
	} else {
		cdtimer = SetTimer("Countdown", 1000, 1);
		cdcount = strval(cmdtext[4]);
	}
	return 1;	
}

Timer:
public Countdown()
{
	new string[256];
	if (cdcount > 0) {
		format(string,sizeof(string),"%d",cdcount);
		GameTextForAll(string,1000,4);
		PlaySoundForAll(1056,0,0,0);
		cdcount --;
	}
	else if (cdcount == 0) {
		GameTextForAll("GO!",1000,4);
		PlaySoundForAll(1057,0,0,0);
		KillTimer(cdtimer);
	}
}

Requires:
Global variables cdtimer and cdcount