How many yards of fabric do I need to make a skirt or dress?

It depends on the pattern, and the width of the fabric.

Your pattern will tell you how much fabric you will need; there is a chart usually on the back of the pattern envelope.

No pattern! Just imagination!

Ok, with no pattern you will have to do the math.
Do you have instructions or someone who has made what you want before?
The easiest to do without a pattern might be a skirt, You would have to estimate the length that you want it to end up, plus extra for a hem and waistband. Also figure out how big around it needs to end up. A full skirt might be easier than a fitted skirt.
Fabric comes in different widths 45 inches is one common width.
Have you ever sewn anything before?

Without a pattern or a type of fabric selected all I can answer is in a very generalized 2–4 yards. If the fabric has a pattern to line up, buy 4yds. If not, you can go with 2.

Wrap yourself up, add a few inches and have the fabric lady cut that amount for you.

You could make a skirt out of a large scarf. That’s only a yard to a yard & a ½ of fabric.

It shouldn’t be hard to find a pattern for what you want (here’s one: link ), but I wouldn’t wing it unless I were an experienced seamstress. It’s not quite as simple as you might guess. For instance, is the waist buttoned, elasticized, or zipped? Different steps for each and different pattern elements.

A ruffled or layered (tiered) skirt is usually full and requires extra material. It’s going to take some extra steps and a lot of extra hemming. You’ll need a seam allowance and/or hem allowance for every ruffle or tier.

If you try to do this on your own, without a guide to help you with order of assembly and so on, you’d better get plenty of extra material to allow for redo.

If I were you I’d definitely buy a pattern. But second best would be to look for a skirt that fits, whether from your closet or from a used-clothing store, and take it apart: use a seam ripper and open it up everywhere it’s stitched, and construct a pattern from that. You can do it in tissue paper or newspaper. Pay attention to how it comes apart—which seam crosses which other seam—and that will help you with order of assembly.

You’ll also need to know how to do gathering. Have you ever done it? It isn’t hard, but it does require a little technique, and the method is much more easily learned than improvised. A tiered, ruffled skirt will involve a lot of gathering.

Lace edging is a nice extra, and that takes a little technique too.

I’ve done quite a bit of sewing! Gathering, ruffles, you name it. Thanks, @SpatzieLover for giving me a number!