Back it On Up!
The Free Option - Time Machine, which comes on your Mac
​Carbon Copy Cloner - Really great option, been around for ever and well designed and configurable.  This is my go-to.
Back Blaze - Ideally you should have a backup at the house/business AND one in the cloud...  This is a good cloud option.

​Parts and Pieces
OWC/Macsales - A great place for hard drives, enclosures and much more, reliable and Mac-centric. 
Monoprice - Really good prices on cables and all kinds of electrical thingies. 
New Egg - Big ol' corporate store, but often better than some of the others in that class.

Secure Password Manager
1password - Been using this for years, really great and helpful. Saved my bacon more than once.
Lastpass - A lot of businesses have used this one, but it has recently had a security leak and has fallen out of favor. I do not recommend.

Will my Mac run the latest Operating System?
With the imminent release of Mac OS Sonoma many folks have this question. See the article below, which answers what the minimum requirements are, not only for Mac OS Sonoma, but many of the previous OSs. 
List Of Minimum Operating System Requirements.

Do I Need Anti-Virus on my Mac?
I get asked this question a lot. Macs have traditionally been relatively free of viruses, primarily because of the architecture, but also because they have been less prevalent in the workplace and have therefore been less of a lucrative target. That seems to be changing slowly, but it is still a rarity to see a virus, or a fully compromised Mac. If you do decide that you need anti-virus then I would recommend looking at the following recent Macworld article which outlines the built-in protections on your Mac and plows through most of the anti-virus offerings out there:
Protections on your Mac and some recommendations

Wireless Stuff
To Mesh or not to Mesh - A good article on the benefits of having a wifi mesh network (versus the more traditional base station and repeater).
Google Nest Mesh - I have this one, have had it for about 3 years and it's been rock solid and easy to use.
Eero - People also like this one... A bit more pricey than the above.

Should I buy a used Mac and not a new one?
This is a tricky question as it really depends on how you want to use your Mac. Generally speaking any used Mac will be behind the curve (now or in the near future) in terms of what Operating System will run on it and all the consequences of that. If you have a heavy use case scenario (such as Film or Audio rendering and editing) my recommendation would always be to get the latest and greatest, because the software needed for those tasks will run best on newer hardware, in most instances.
Here is a great Resource for an overview and comparison of all the latest prices on Macs.
However, there are many examples where a used Mac will be sufficient for most people's needs. In the end this is a personal decision and I would highly recommend that you do some good research and think about what your personal needs are. Having said all of that below are some good places to look for used Macs.
Apple Refurbished Macs Refurbished Macs from the mothership.
Macsales Used Macs Reputable store for used Macs. Good support.

Apple Chargers are Expensive. Do I need them?
No, but you have to be careful about what kind of chargers and which wattage is required by your Laptop or Phone.
Here is an Apple page for what's required by Mac Laptops
Mac Laptop Power Requirements 
​And here's a recent good articles breaking down the best non-Apple chargers:
Macworld Chargers Article - Note that this article covers mainly iPhone chargers, but look closer and you will see that there are some laptop options in there as well.

Which Hard Drive Should I Buy?
So essentially there are two different kinds of drive. The older style, which has moving parts (known as an HDD) and are spinning magnetic discs that data is recorded on (and are slower) or the faster, more efficient Solid State Drives (SSD) which have no moving parts (and are more expensive). It really boils down to what you need your hard drive for. If its for backing up and archiving data you may want to consider an HDD, but if its something you are going to be using as an active secondary data drive, you will probably want to use an SSD. In this day and age the price difference is not that great, so I usually only buy SSDs at this point, and would recommend those overall.
Here are a couple of options that I use and have been good to me:
Kingston XS1000 SSD -  Somewhat bigger than a Thumb Drive, small, very portable and good for the price.
Samsung 1TB T7 S SD - Small, lightweight, more rugged than the above, good portable option
Lacie 1TB Rugged SSD - These are a workhorse, but more expensive. 
Western Digital 1TB HDD - This is a good no-nonsense option for backups at an affordable price. 
Here is a good overview of SSD options that ae good for the Mac.
Also check out the options at OWC/Macsales. They have quality products, but can be a bit more pricey (although not as bad as the Lacie)

What about monitors?
Monitors really are an item that varies from person to person and from workflow to workflow. The monitor, more than almost anything else is a personal preference, as it is what you actually see, what you are physically looking at. So size, clarity, color calibration, aesthetics may all come into play. I can point you in certain directions, but I highly recommend you do some  further research, and make absolutely sure that what you are buying is appropriate for your needs and machine.
This Philips monitor has been getting good reviews and is worth considering as is this View Sonic monitor.

Mac Info and What's New
Hard to say that any one of these is better than the other. In fact you'll see the same info (and rumors) on most of these pages....
Mac Rumors - This one has a good buyer's guide, i.e. when the last refresh was and whether its wise to buy now.
Apple Insider
Macworld
Cult of Mac

PRAM/NVRAM and SMC - Huh?
This article explains it well.  Know why you're doing this before doing it. Talk to me if you have questions.