Measure twice, cut once.
A measuring tape is an indispensable tool. A good measuring tape is a joy to use. There are a lot of crappy measuring tapes.
You should be using your measuring tape on almost every project. I know I should. And I know I don’t. But I know I should; so do as I say, not as I do, and measure everything: Measure the hole before you cut the patch. Measure the patch. Measure the empty space before you go buy the new furniture. Measure the furniture before you put down your money. Measure the openings on your car to make sure you can get the furniture into the car to get it home.
Buy a measuring tape more than long enough to measure what you need; there’s nothing more annoying than discovering the thing you wanted to measure is just a little bigger than what you can measure. I have a selection of longer and longer tapes purchased as my projects got bigger, ending with a very nice 40-foot tape. You probably don’t need a 40-foot tape to start with; it’s fine for smaller measurements, but it’s heavy, and it was not cheap. I’d start with a 25-foot.
Pull it out a couple-three feet. Let it snap back. It should come out smoothly (with no “scrapey” feeling) and slide back in with some authority. Try out the lock button. I’ve owned cheap tape measures where the lock button was hard to engage and/or hard to disengage. That’s bad. Feel (carefully) the edge of the tape. It should feel substantial. It will be sharp enough to cut you if you’re not careful, but how sharp is a good indicator of how thin and flimsy the blade is: the thinner the blade, the thinner and sharper the edge.
Some tape measures brag about their “standout”. The standout is basically how far you can extend the blade straight out horizontally in mid-air before it falls. Which is useful. But not so useful you should prefer one that quotes a larger number over a better-made one with a smaller standout.
Now that you have a tape measure, treat it well. Do not bend it closer than it naturally wants to bend. Do not bend it backward on purpose. Do not let the return spring reel it all zooming back in as fast as it wants. a: This can damage the spring, the blade, or both, and b: The blade is sharp and you don’t want it moving so fast it becomes a linear meat slicer. Modulate the speed with the lock button or your hand (but if it’s already going too fast don’t touch the blade and cut your fingers off). If you notice the slightest crease, fold or crack on the blade, replace it right away; no one wants a jagged blade on a heavy spring flailing around. Don’t let the thing get too wet or it will rust inside.
So go measure the world.