OK - when you say you can't get it to fit, what do you mean exactly?
For info, the process should be this:
Create a new image. Set the DPI / resolution to be 300dpi (or 600 if you have a printer that can print that high, and the images you want to print are very large), and a document width and height the dimensions of your card. You can set this by changing the drop down menu from pixels to mm or cm if it isn't already. Click OK. This should now have created a blank document, in the exact dimensions of the card once it is physically printed.
Now open your image file. Use the largest version of the file you have. Using the Move tool, drag your image onto your blank document image.
Now, if your image once you've dragged it across appears much smaller than your blank document (half the size or less), then you might want to consider starting again, and using a smaller resolution (if you had it at 600, go to 300, or if you had it at 300, go to 150). Then repeat the above process.
If your image appears massive, and doesn't fit on the blank document, then we're in a good place.
Select the Marquee tool (the dotted line box), ensure your photo layer is selected in the Layer palette, and right click on it. Choose Free Transform. Now Zoom out of the document by pressing Ctrl + the minus key (or Option + the minus key if you have a Mac).
You'll see your document zoom out, and the box around your photo sitting on top of it. Now make sure the top left corner of the photo is sitting in the top left corner of the blank document. Hold down the shift key, and drag the bottom right corner of the photo inwards. you should see your photo reduce in size. Keep doing this until either the width of the photo matches the width of the blank doc, or the height matches the height of the blank doc (whichever comes first). Double click inside the box surrounding the photo to accept the transformation.
If your image is now cropped on either the horizontal or vertical, you can use the Move tool to move it around to get the best crop.
I hope this helps
