Depreciation--Str Line Or Decl Balance???

Need some help understanding depreciation: 1) When do I use Straight Line Versus Declining Balance depreciation? What are pros and cons of each? 2) Over what time period do1 I depreciate each? Is Double Declining an accelerated schedule? 3) When I make capital improvements to property do I start a new deprc schedule for the incremental capital? 4) I am looking at rental properties, how do I determine the property value to depreciate of Furnishing and Fixtures? Any advice will be appreciated.