Ruric Thar's triggered ability triggers whenever any player casts a noncreature spell, including you.

Ruric Thar's triggered ability resolves before the spell that caused it to trigger. The ability will resolve even if that spell is countered.

If Ruric Thar can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having a creature attack, the player isn't forced to pay that cost, so Ruric Thar doesn't have to attack in that case either.