Are you prepared?  It’s a simple, devious question.  Most folk can mostly deal with the mundane stuff they see day to day, but, depending on the person, are less able to deal with the unexpected and unusual.  When life throws a weird situation at you three things will resolve it; will, ideas, and tools.  Will is the most important, the hardest to measure and instill.  Do you throw up your hands and panic, or work with things and solve the problem?  This, you need to figure out on your own, practice and experience will your best teachers.  Ideas, (concepts, techniques, and resourcefulness), are the next most valuable, as they are take up no space, have no weight, are wear proof, and (according to Alan Moore), bulletproof.  They cost only the time it takes to learn them, (remember, time is your most valuable, fleeting asset).  Tools can’t be overlooked despite their lackluster place on this list.  Actions impossible barehanded become easy with the right implement, we’re a tool using species.  This makes tools indispensable if you know what to do with them. 
People’s takes on what is necessary to know and carry will vary WIDELY, but here’s a rough starting point based on various life and work experience in the city, country, and on the ocean.  These aren’t meant to make you an expert survivor or engineer overnight, but are handy things to know for situation from the minor inconvenience to difficult pinch.  Adopt what works for you, get rid of what doesn’t.  Choose wisely.

5 Useful Things to know:
1. Surroundings:  What is there / happening?  What ISN’T there / happening?  How does it work?  Be aware, size things up, and absorb.  Make it a game.

2. Tool use:  Know how to hammer, pry, screw, or bend things.  Know how to safely and effectively cut, whittle, trim, or prep food with a knife. 
   
3. Fire starting:  Read up on methods and play around to get the hang of them.  Tinder, dryer lint, liquid / solid / gas flammables, napalm, etc.  If nothing else, this makes you look proficient at cookouts.  (Watch your eyebrows!) 

4. Basic Sewing:  Knowing a few basic stitches turns torn clothing from a bad situation into a solved one.  Beyond that, knowing how to manipulate flexible expanses of cloth, hide, or plastic opens up whole realms of improvisation.  Learn knots for thread and rope too.

5. First Aid:  Learn how to clean / close wounds, treat shock, severe allergies, hypothermia, stop bleeding, etc.  You don’t need to be a field surgeon and you SHOULD NOT try to be one unless medical care is a far off hope.  You may need to go beyond basic first aid courses for meaty info, as they defer to the medical system whenever possible.  Advanced Courses or First Responder training are good to have.

5 Useful things to have:
1. A water bottle:  Thirsty?  You will be.  Keep one around and fill up whenever possible.  Dehydration SUCKS.

2.  A knife:  Trust me, you want one around as legality allows.  They are very useful tools.  What knife to carry is a long discussion, but for MOST daily utility uses a well made folding knife, or Swiss Army knife (provides other options), will do. 

3.  A lighter:  Even if you’re not a smoker.  Think of all the things that require or can be helped by application of direct heat.  Fire sterilizes things.  Remeber our cookout example?  It beats knocking rock together should you need a fire. 

4. Sewing kit:  with a few sizes of needle and types of thread, this can save your ass (from exposure caused by an embarrassing rip in your pants).  Yes, you COULD improvise sutures, but leave that to the pros whenever you can (it stings).  Beyond that, you can floss or tie small items with the thread, or use the needles when small scratching or piercing is needed (like removing splinters).  Add some parachute cord to this kit for greater flexibility.

5.  Small first aid kit:  Even an empty mint tin stocked with disinfectant swabs, small bandages, antibiotic cream, antihistamine, a few aspirin, and some tape will NEVER be useless.  People always get hurt.

This list hasn’t covered things like clothing or shoes, which are best adjusted by the individual depending on area, exposure and occupation.   Specific advice for unique situations will follow, but improvise for fun when you can to try out what you know.  Add and remove items as you find need or lack thereof.  This additional practice and knowledge will just build upon and compliment what you already have.