Just bought a KLR from Craigslist and the guy seems pretty honorable about the state of the bike, but I’m wet behind the ears when it comes to motorcycles. What’s the first thing I should go through to make sure I’m ready for any and all situations concerning the mechanicals? What tools should I have/get so I can work on it?

Go over all fasteners for proper tightness. inspect for any damage to bike. Don't forget underneath the bike, wheel wells, etc. Change oil, and filter. Check air filter, replace if necessary. Learn how to remove the plastics.
Inspect tires, check for proper air in tires.

Collect proper tools to work on bike. ( I like to make up a tool kit that has any tool necessary to do field repairs.)
If putting a bike tool kit together, look for just the sockets, wrenches sizes that your bike needs. You don't need a complete set of wrenches, sockets if the bike only needs 1 or 2 sizes. Why carry extra weight, or take up extra space, if you don't need to? Home Depot, Lowe's sell individual sockets, wrenches.

All this will give you a good basis to know your bike. You'll be better prepared to work on it, or even explain to your mechanic what's wrong if need be.

If you plan to work on your own bike, arrange for a way to lift it. A cheap motorcycle lift can be bought from Harbor Freight. Some OEM skid plates don't mate with lift jacks. I bought a better skid plate so it would mate with my jack. ( Flat bottom. )

If bike is new, do proper break-in. If used, keep record on how many miles you can go on a tank of gas.
I plan to carry a small amount of gas, and run my new XT250 tank dry, to see how many miles it will go after going on reserve. Good to know , just in case you run short.

I'm sure someone will mention the Doo-Hicky. Probably a good idea to find out if it has been done.
Follow this forum for other general information. A real good source of info. Some things mentioned will depend on how you plan to use the bike. ( Some demand more performance, then others. ) Hard off road is quite different from general back road riding, or commuting.

Ride safe, and enjoy!

@Dudordude. Welcome to the forum and congrats on the KLR purchase. What year is the bike.

First thing you want to get a hold of in the way of “tools” is a Clymer Repair Manual for the bike. From there, along with the resources of this forum, it’s easy. We’ll get rid of that ear wetness no problem.

PS - Please add your location to your forum profile.

Do Not Loose or Forget the thick aluminum engine oil drain plug gasket, incompletely cut threads on a drain plug can crack the oil drain boss & create a leak.

The KLR air filter Must Be Re-Oiled After Cleaning!!!

Have you read this yet?

http://www.klrforum.com/introduction...kes-avoid.html

Get a Clymer manual and/or the Kawasaki Service Manuals. Not "if" but "when" you seek advice on a forum such as this, it makes it incredibly difficult if not impossible for folks to help you if you do not have a manual to refer to.

As for tools, I'd recommend Harbor Freight as a good source of tools for the Do-It-Yourself person. They're inexpensive, reasonably accurate and durable enough. They may lack the lighter weight and low bulk of say Snap-On or Mac Tools, but as long as you're not trying to make your living with them . . . . they'll be more than sufficient.

It would be wise to include Inch Pound and Foot Pound Torque Wrenches for your "at home" service work. KLR nuts, bolts, plugs and other fasteners do have a habit of coming loose over time due to the vibration of the engine. This is normal. HOWEVER, many of these nuts and bolts, such as the Oil Drain Plug, have an extremely fine line between being "tight enough" and "Oh Sh*t!"" stripping the threads. Particularly if you are new at this (and even if you're not) you will absolutely need these Torque Wrenches, set to the specification of the fastener in question, or your "experiment" can and will cost you many, many, many times the price of the Torque Wrenches.

Lastly, give a location and the year of your bike in your signature or in the information that shows with your Avatar. Like having a Manual, it helps everyone else to help you.