I do generally recommend reading the Sharpe books in chronological order. Click on the Sharpe books page of this website for a complete chronological list of the series.
Ensign: Lowest of the low, doesn't really command anything except at the whim of his superiors. In the Rifles he's called a Second Lieutenant.
Lieutenant: Still fairly lowly, subordinate in a company to a Captain
Captain: Probably one of the best ranks. A captain commands a company (each battalion is divided into ten companies, so a company will have anything from 50 to 100 men)
Major: Getting impressive now. Ostensibly a Major commands half a battalion, but they're really there to assist the battalion commander and be ready to take over if he's killed.
Lieutenant Colonel: He commands the battalion. He's god.
Those are the officers with the battalions, but above them are:
Colonel: He's a bit of nothing really. Not senior enough to be a general and too senior to command a battalion, but he might well be given some specific responsibility - commanding troops from more than one battalion to guard an outpost or attack some small objective.
Then come the Generals - in ascending order, and the higher you get the more units might fall under your command.
Brigadier General
Major General
Lieutenant General
General
and at the very top
Field Marshal - who really is god.
Hope that helps!