From my experience, I would lean more towards pants/skirts/capris and leather shoes than jeans and sneakers, and definitely stay away from baggy tshirts and shorts if you don't want to stand out. Tank tops are fine for everyday, just not in cathedrals and such. Spanish women are very stylish and tend to dress up a bit more than Americans, a more tailored look. Most women your age usually have on a simple top and pants (if you really wanted to look Spanish, you would wear your pants a size smaller than normal - they wear them tight!) or skirt, sundresses are good, too. Definitely check out the stores mentioned earlier (Zara, Mango, etc), they are all over the place and it seems like every store in Spain is having rebajas right now, so you can find some great deals! Have fun!