Basic bike maintenance

Depends whether you're time-rich or not. If you don't clean your drivetrain regularly it will wear out and need replacing more quickly than if you do.

Replacement cassettes and chains are easy to fit and will take a lot of abuse, particularly if you run a heavy wet lube in the winter months. You're looking at £50 and an hour to fit for a replacement cassette/chain versus say 30 minutes a week to keep everything spick and span.

Keep an eye on the pad wear as that's critical to safety - and consider swapping pad compounds if it gets really wet before you're back in your car.

You'll probably find that your shifting will go out of whack in a few weeks as the cable outers bed down into the ferrules. Manifests as a slow shift and usually fixed by a quick (and small) tweak of the barrel adjusters. Do it in half-turn increments and test under load (with you riding, rather than on a workstand)

Mudguards if you can fit them go a long way to keeping you and your bike as free from grime as possible. 

Ditto chain.
Lubes such as pictured are awesome. Different colours to suit conditions. Eg. Dry, wet, a bit of both.. Whatever suits you.
LBS here has RockNRoll and Muk Off. I'm sure there's probably others.
Basically clean chain, cogs, cassette. Apply lube, wipe off excess (the lube's designed to penetrate near fulcrums of chain).
The screws on the back of the derraileur are the outer and inner limits of the cassette. When shifter doesn't shift properly (the step adjustment), that's the connector at the end of the cable. The screws let the chain know the boundaries, the cable itself has a tension adjuster that allows the control of shifts. The tension is usually the adjustment bit.
Lubes like those are awesome

Steering, stopping, propulsion. 

So pay attention to handlebars for smooth non grinding motion. 

Check brakes over once a week for wear or unusual noises.

keep chain and gearing clean and lubricated, especially on wet muddy days.  

Get a set of mudguards, it will keep you and the bike much cleaner.

also get yourself some decent lights, may two sets.  I like NiteRider 850 front and they do a nice bright rear light too.  They are rechargeable, so have a charger available at work and home.  

The chain, as others have said.

If you have clipless pedals, keep them free of mud and crud (and the cleats as well, of course!)

Keep an eye on the brake linings. If you have rim brkes,clean mud and grit of the rims and the brake pads.

Try these vids from GCN they are generally a pretty good start point for learning how to do most things about looking after your bike.

https://www.youtube.com/playlist?list=PLH49XyD2ofsXzMTbrSNLAXnQo0pSyDEb6

Ditto about the drivetrain. You can never spend too much time caring for your drivetrain.

Check your tyres have sufficient pressure every so often as well as that can make a big difference to how fast the bike goes and may provide warning of a slow puncture.

If you're going to use a pressure washer, try to avoid shooting water at any of the bearings as the pressure can allow water to get past the bearing seals and then cause corrosion. Personally, I use a bucket with warm water and car shampoo (doesn't have salt in like washing up liquid) and some old washing up sponges when my bike has some dirt on it (probably every other week).

In general, bikes don't require much maintenance, but pay attention if a new noise develops whilst riding. Although noises are notoriously difficult to pin down, they often signal that something has worked loose or needs attention.

The most important thing is to keep the drivetrain clean.  The rest doesn't really matter, that's just aesthetics.  But the chain, a very good wipe down (at the very least) and lube once a week might do you, but depending on the filth maybe more.  If  it's especially grimey, a chain cleaning device once or twice a month, and a strip down every couple of months.  You will then start needing tools.

Enjoy the obsession.