Man, if you're starting out and you know you'll need several components for a bag... just get this. It will save you so much time picking everything out that even if you balk at the price (amazing value, by the way) I bet that if you did the math using your hourly wage and the time spent picking everything out and making sure it fits... you'd save a lot of money and a lot of irritation if you just got this. Just get this.
But before I get started, let me share with you a few tips. OK, one tip: Get a work stand. Because putting your bike upside down isn’t good for it—yes, I said it. When I first started fixing my own bikes, I couldn’t afford a fancy $300, folding, magnetic, fixes-the-bike-for-you stand. I tried piecing together a “DIY $30 Stand.” I followed all the directions that the Internet demanded, and once I’d bought all the clamps and pipes, this thing cost really more in the neighborhood of $60—but it only stayed upright with a rope tied to something sturdy in the garage. It was one word: junk. So I bit the veritable chainring and charged a decent stand on my credit card (I hate debt, folks).
These guys are awesome. After work, I got to my bike to discover my back tire was completely flat. It had a slow leak, so I was able to pump it up enough to make it to their shop. I got there 10 minutes before closing and they were able to fit me in. They replaced my tube and picked all of the glass shards out of my tire. Awesome customer service and they saved me from having to do this myself in the dark and the cold.

Let me go ahead and get the bad stuff out of the way. First of all, this stand is NOT the same level of quality as something you would find in a bicycle shop being used for the shop's repairs. it's quite a bit more flimsy, and the grip likes to rotate if you don't get the bike's center of gravity just right. The tool tray is also made out of a rather cheap plastic that can break easily at the rim. That being said, this stand is perfect for home users and makes mundane tasks like fixing a flat or cleaning the chain much easier. It easily holds my Raleigh Merit 3, and I don't worry about the thing falling even if the grip tilts a bit. The stand significantly shortened the time I needed to change an inner tube on my bicycle, and made me less worried about damaging something on the bike by jury-rigging a stand out of whatever I had lying around.
On a very sunny Saturday I had a rear spoke break - you can imagine that no shop was willing to promise a quick repair! I was so fortunate to find this shop and will come back - Andy not only told me to come down and he'd get it fixed, he had his assistant work with another couple while he measured, cut to the right size, installed and trued the wheel while I waited! It was awesome, and the whole place had a great feel and good people. Best of all he charged a very fair $20 for all that work - I would have paid a premium for the quick service! After experiencing lots of attitude at other shops and poor service, this was a real find. I hope you will check them out, they have a great selection of bikes and accessories too! Good people and good craftsmanship should be rewarded.
If you’re going to use the chain whip you just made, you’ll also need a way to get that cassette off. A lockring remover is a must-have; unless you use one of the shady removal methods I’ve seen on the internet—which I wouldn’t necessarily recommend unless you don’t care much about the condition of the wheel after you use them. I recommend Park Tool’s iterations here that cost about $6, but you’ll need to buy the specific one for your brand/type of cassette. (By the way, I’m assuming you already have an adjustable wrench laying around somewhere to use with this tool. If you don’t, well, you’ll need one of those too.)
Brakes come in a variety of designs depending on the age and style of your bike. Rim brakes offer more modulation but have less stopping power, so are more suited to riding on the roads. Disc brakes can provide huge amounts of stopping power so are best suited to off road riding, although ‘V’ rim brakes are also well suited to mountain biking. All brakes use a friction system to stop the movement of the wheel, so regular checks of the contact pads should form part of your regular cycle maintenance.