// in OnPlayerCommandText  (if you're using that??)
if(strcmp("/commands", cmdtext, true, 8) == 0)
{
    // using 1 for dialogid might cause conflict, change it to the next available id.
    ShowPlayerDialog(playerid, /* dialogid */ 1, DIALOG_STYLE_LIST, "Commands Menu", "Player Commands\nAdmin Commands", "Select", "Cancel");
    return 1;
}

// if using ZCMD

CMD:commands(playerid, params[])
{
    // using 1 for dialogid might cause conflict, change it to the next available id.
    ShowPlayerDialog(playerid, /* dialogid */ 1, DIALOG_STYLE_LIST, "Commands Menu", "Player Commands\nAdmin Commands", "Select", "Cancel");
    return 1;
}

// In OnDialogResponse

if(dialogid == 1 /* or the dialogid you selected */) 
{
    if(response) {
        switch(listitem) {
            case 0: {
                SendClientMessage(playerid, -1, "*Commands* Some commands here");
            }
            case 1: {
                if(IsPlayerAdmin(playerid)) {
                    SendClientMessage(playerid, -1, "*Admin Commands* some commands here");
                }
            }
        }
    }   
}