new Announce[][] =
{
    "testmsg1",
    "testmsg2"  
};

// Will be at OnGameModeInit/OnFilterScriptInit
SetTimer("AnnMessage",60000,1); //60 seconds timer

// Place it anywhere
forward AnnMessage(playerid);
public AnnMessage(playerid)
{
        SendClientMessageToAll(-1, Announce[random(sizeof(Announce))]);
        return true;
}