Depth of yellow field - see what I did there?

For my birthday last year, my lovely husband (he's mentioned a few times that this is becoming a Liam-bashing blog so I'm trying to be nice) bought me the camera I had been lusting over for months. I wanted to learn all the fancy techniques now available to me, but almost a year on, I'm not sure I've got very far - I'm feeling I'm still only skimming the surface. And this picture is a case in point.

Now we live in the country, everywhere I go involves driving through fields and fields of crops, and right now, the oilseed rape flowers are starting to come out. I love yellow flowers so on Sunday on the way back from Bradwell, I asked Liam to pull over so I could take a picture.

One of the things I've learned about since I got my camera is depth of field - how much of the photo in front and behind the subject is in focus. I'd never heard of this before, but I love how this can alter a photo - making people stand out from a background or showing more of their surroundings. The photos below aren't mine - this is my problem - I can't figure it out how to do it!

I want to be able to control this! I've read about it - I know its to do with smaller and larger f numbers - I see them on my camera, but I still don't get how you do it!

I tried to get all of my oilseed rape picture in focus, but I could only manage the narrow band across the middle. I used a few different setting, but they all came out the same - blurred at the front and back. I did only have a few seconds to try, cause I had a car full of freezing and cranky kids (and husband) but this is my goal for the week - hopefully I can figure this out, then on to the next photographic hurdle! Any advice?