Make sure you know Ben Thanh bus station ( opposite to benthanh market). In here, catching the bus number 703( tour from benthanh to Moc bai), this bus will stop at Moc Bai bus station. And in here let catch the bus number 05 to Tay Ninh. You could catch this bus in Ben Thanh at 6AM - 16PM.
Another tour is the bus 13 from Ben thanh to Cu Chi, 94 if you are at Cho Lon bus station. You could take these bus to Cu Chi and continue catch the bus number 603 from Cu Chi bus station to Tay Ninh.
And one more way you could be there easier and exactly where you want to go to, let go to Mien Dong bus station (bus 26 from ben thanh to mien dong) and come to ticket room and buy a ticket to Tay Ninh, only 40vnd-45vnd. they would take you to the place you want in Tay Ninh.