Holes in my hat every time

Hi,
I’m new here and have just started crocheting again.
I have a problem: I want to crochet a hat, but it keeps coming out with holes. I’m using half double crochet stitches.
What am I doing wrong? I’m following the pattern from YouTube.
See the photo.
Thank you so much for your help.
Best regards, Nata 

Hi Nata,
the color gradient is beautiful. You might want to pull the magic ring in the middle—the ring at the beginning—a little tighter. Maybe it would help if you pulled the stitches together a bit more tightly? To me, they look a little uneven and too spread out in the photo, but that might just be because of the photo.

Are you sure you only have one yarn over between each stitch? It looks like there are more here and there, but again, that could just be the photo. That might have caused the slightly larger gaps, though it’s relatively small for two yarn overs. The experts will surely have something to say about this—maybe they know more.

You’ll get the hang of the hat—if you’ve crocheted before, you’ll get back into the swing of things quickly. Just keep at it, and you’ll be back on track.

Thanks for your reply.
What do you mean by “yarn over”?

Hi Nata,
You have a stitch on the hook; make a yarn over, insert the hook again, pick up a loop, and pull it through all 3 loops. But you have to pull it really tight so that no gap forms. It’s just a matter of practice—you’ll get the hang of it in time.
Anyway, have fun crocheting. Helga54

Maybe try using a smaller crochet hook...

Best regards, Nici

Thanks for your replies.
I tried to put everything into practice: a smaller hook, tighter stitches. It already looks quite a bit better. Now I have the following problem: the circle doesn't lie flat, but is wavy. I've already started over 3 times now. Always wavy...

You might have increased too much too quickly. On the 2nd round, double every stitch, then every 3rd, and so on—or you’ll
need to find a pattern. I always crochet hats using a basic pattern; I find that easier to follow.
Give it a try—it’ll work out. Have a nice Sunday!

I crocheted following the pattern in a YouTube video and assembled it just as shown there.
Well, I’ll just keep crocheting and see how it turns out.
Thanks for your replies!