Combat problemsFollow

I'm new to the game, 2 days, and I am having problems with the combat, targeting and interaction with NPC's.

When I target a mob or NPC most times it will tell me I am too far away, even though I am standing right on top of them. Then after a few seconds they disappear and reappear somewhere else. Sometimes I can't even see a mob that is attacking me so I have no way of knowing which direction to face. I have tried to use the "stick" command but sometimes the mob will wander off and I still cannot attack it.

My computer is fairly nice so I have no doubt the hardware isn't the problem. Any ideas?