Working on your car is easier if you have the right tools. In fact, there are a lot of things you can't do without the correct tools. For example, if you're going to gap a spark plug, you need a spark-plug gauge to check the gap. If you're checking a spark plug wire to see if it has the specified resistence, you need an ohmmeter, and so on.
On an elementary level, you can't tighten a Phillips screw with a screwdriver made for plain slotted screws, and vice versa.
In fact, to do any work at all beyond a visual inspection for problems, you need a basic set of hand tools. Most hand tools will last for years, maybe even for a lifetime. So the investment more than pays for itself over the years.
A basic set of hand tools consists of:
Open-end and box-end wrenches that range in size from 5/16" to 1 inch. Although there may be occasions when a smaller or larger wrench is needed, this range of sizes will let you handle most fasteners you're likely to encounter.
Wrenches are available in various configurations. You can get a wrench that is a box-end wrench or both ends; in this case you have a wrench that will fit two sizes of fasterners. One end, for example might be for 1/2" fasteners, and other end for 9/16" fasteners.
Or you can get an open-end wrench with two open ends -- as with the box-end wrench, one open end fits a smaller fastener than the other.
Or you can buy combination open-end box-end wrenches. In this case, both ends of the wrench fit the same size, but one end of the wrench is an open end, the other a box-end.
A box-end wrench is less likely to round off a nut or bolt head (if the wrench being used on the fastener is the proper size) than is an open-end wrench. But there are times when a box-end wrench won't work because you don't have the space to get it on the fasterner.
In any case, be sure the wrench you are using is the correct size. It can't "almost fit." It must fit exactly. Otherwise you'll very likely damage the fastner when the wrench slips.
If your car has both metric fasteners and standard fasteners, as many newer cars do these days, then your life is complicated even more. You'll need a set of metric wrenches as well as a set of standard wrenches. With metric wrenches, a set of wrenches that range in size from 6mm to 24mm will handle most of the metric fasteners the Saturday mechanic will encounter.
Don't try to use a metric wrench on a standard fastener, or vice versa. Some sizes of metric and standard wrenches are very close -- but they're not the same.
How do you tell if a fastener is metric or standard? Sometimes the only way is to keep trying wrenches until you find one that fits exactly (with experience you'll be able to eyeball a nut or bolt, and more often than not, pick the wrench that fits exactly).
Some cars with metric fasteners and standard fasteners in the same car may have the metric fasteners marked. Many newer GM cars have many of their metric fasteners colored blue, for example. Check the shop manual for your car, or ask a mechanic or service manager at a local garage about your car.
Screwdrivers -- plain, and Phillips. Buy both Phillips and plain screwdrivers in various sizes, from small to large. This way you'll be able to handle most any screw you need to loosen or tighten.
Be sure you have the right size and type of screwdriver, when loosening or tightening a screw. If the screw head is filled with dirt or grime, remove it with a piece of wire or other suitable object before trying to use the screwdriver. If the screwdriver can't fit into the screw all the way, it can slip, damaging the screw head.
Don't try to turn a screw with a screwdriver's that's too large or too small. This can damage the screw head too.
Ball-peen hammer. There are times when a hammer is necessary. Don't use an ordinary claw hammer made for driving and pulling nails. Use a ball-peen hammer. Whenever you use a ball-peen hammer, wear safety glasses.
Slip-joint pliers. These are handy for holding things. And, as the name implies, their joint (where the jaws pivot) can be adjusted in two different positions -- to hold smaller and larger objects.
Don't use pliers to loosen or tighten nuts and bolts. They aren't designed for that. But there are many situations when pliers are indispensable. No tool box should be without them.
These tools are the basics. There are plenty of other hand tools, diagnostic meters and other items that will let you do even more work on your car.