The labels on Spanish wines aren't always as good an indication of the quality as you'd like them to be but look for 'Crianza', 'Reserva' and 'Gran Reserva'.
One of the best wine regions in Spain is 'Rioja' (which gives its name to some superb reds) but the same grape is grown outside of the region and produces some excellent wines which can't be labelled as 'Rioja'; simply look for the name of the grape: Tempranillo.
Spain doesn't produce many good white wines. Look for the grape 'Verdejo' if you're trying to find one of the better ones.
Also, see here:
http://www.thewinedoctor.com/regionalguides/sp ain.shtml
Chris