Shutter speed, aperture and ISO...trying to understand it all

[Bekkkk]

I'm getting my first DSLR next week (Nikon d5000) and I have a friend's showjumping comp that I said I would come take pictures of, (I usually just use my point and shoot but its only a casual thing so I thought I may aswell try out my new camera) I have been reading a lot about exposure I think I am getting it..

Correct me if I am wrong, I am trying to get my head around all this. So because it's at speed I want a fast shutter speed to freeze the shot. If I use a fast shutter speed, then I am going to have to have a big aperture so that there is enough light.
I want the rider, horse and jump in focus and the rest out of focus. So I change the aperture to achieve this. How do you know what fstop to use? Do I just take a picture to see what is in focus and go from there?
So I should use the fastest shutter speed and the aperture that gets the depth of field I want. If I do that and then the picture is too dark I change the ISO, but not too high ISO otherwise there is noise. Does this sound right?

[OsmosisStudios]

Originally Posted by Bekkkk

I want the rider, horse and jump in focus and the rest out of focus. So I change the aperture to achieve this. How do you know what fstop to use? Do I just take a picture to see what is in focus and go from there?
So I should use the fastest shutter speed and the aperture that gets the depth of field I want. If I do that and then the picture is too dark I change the ISO, but not too high ISO otherwise there is noise. Does this sound right?

I think you've got a bit of confusion here. Aperture doesn't determine WHAT is in focus, but HOW MUCH.

Depending on what lens(es) you have, you may not have much options in this.

The rest of your reasoning is sound. Try using the S (Shutterspeed Priority) mode for starters: that lets you choose the shutterspeed you want and let the camera do the rest of the work.