If you got gift cards to iTunes, but do no own an Apple brand iPod, you can still use the card to buy your music and put it on the device, it's just an annoying time-consuming process.
I used to have a Creative Zen mp3 player, and this is what I did. Download itunes from the Apple website. Set up your account and all that. Use the card to buy whatever music you wish. It will then appear in your iTunes library. Then, you need to "convert" your music ... because iTunes has its music encoded as a certain type of file which is not an mp3. To do this, select all the music in your library (press ctrl + a) and then go to the tab up top that says "Advanced". I believe it now says something about create AAC version... it used to say Convert to mp3. either way, once you choose that option, you will notice at the top center of your itunes window, that it is converting each song, and this will cause you to have duplicates of each file. You then can transfer to converted song (the second one) to your mp3 device by burning a CD of the songs you want to put on there (which is a good back-up, since then you'll always have the CD if your itunes library or other music library crashes, etc). Or you can put the songs on a flash drive and transfer them to the mp3 player that way.
Good luck!