creating a layer mask in photoshop 7

When I try to create a layer mask in Photoshop 7. what happens is every time I use the oval marque tool to select the area to keep it says something about 50% pixels being selected and when I hit the layer mask thumbnail it doesn't select what I want. How do I make a layer mask effectively?
nmystic

Is "feather" set to 0?

No feather was at 100 and would you believe non of my 4 books told me I needed to do that. I tried setting it to zero and it worked!! thank you so much I can't tell you how frustrated I was. You are a genius!
take care
nmystic

You're welcome.

No genius, just been caught that way myself in the past........