I don't think that you can shield bash in Oblivion like you can in Skyrim. Instead, your power attacks stagger enemies.
From memory, power attacks are done in the same way as they are in Skyrim - hold the attack button. As a PC/PS3 player, I'm not sure how this is done on Xbox - I'd assume from your post you just hold down RT? Like in Skyrim, this is governed by your fatigue, meaning you'll need enough fatigue to perform a power attack.
As for your last point ... All that I can say is turn down the difficulty as needed, and keep your equipment up-to-date. Most of us around here play with the difficulty bar set to about half way - not insanely difficult, but not too easy. As for equipment ... Everything is strictly controlled by your level. As you gain levels, lower-leveled enemies (for example rats) will disappear and be replaced by more powerful enemies, such as minotaurs. Be sure to keep your weapons up-to-date with your level. Bandits dropped leveled weapons, making them a good source of weapons.
Good luck - and welcome to Cyrodiil!