How do I tell what needs repaired/replaced/cared for?
Clean and oil the chain regularly.  A "chain washer" (about $30) is good for cleaning while making minimal mess.

Since you have to store your bike outside, use "wet" chain lube (at least in rainy season).

Buy yourself an inexpensive "chain wear indicator" for about $10.  Replace the chain when it's stretched to about 0.8, and, when you replace it, check your cogs to see how worn they are.  (The Park Tool site likely gives some hints as to how to check for cog wear.)

Visually check your brake blocks every now and then (assuming rim brakes), and replace before the metal part comes close to touching the rim.

Occasionally inspect your cables for fraying or excessive rust, and replace if indicated.  As the bike is stored outside, occasionally put a few drops of lube on the cables where the inner cables slide into the outer housings.

Occasionally inspect your tires for wear, and for cracks in the sidewalls that indicate they are "drying out".  (And, of course, get a good "floor pump" with built-in gauge and check your tire pressure at least weekly.)