first of all, i am not sure whether getting married here will immediately allow you to stay. you may need to get married, then go home and ask for the visa for persons married to spaniards and then return. plus, i don’t know how having stayed illegally will reflect on your visa application. they may not even notice, but they may freak. try to smudge the date stamp on your exit (i highly doubt the blas? passport stamper in the airport will notice that you’ve overstayed your welcome if you’re american. they often just wave you through at Barajas without even looking at the inside of the passport)
as for the marriage process itself, it is long and EXTREMELY bureaucratic. i just went through it (american married to a spaniard, as well). the most difficult thing for you to get will be your birth certificate with the Apostille of the Hague on it. in California, this meant going to the county to get a “real” birth certificate (the one i had didn’t cut the mustard for Spanish authorities, even though it had my footprint and a gold seal from the hospital, plus my doctor’s signature, and more), then that “real” birth certificate has to be taken to a place that puts on Apostilles (there are only four in all of california and it costs 200 dollars)... of course, your parents will have to do all this for you, since you can’t go home and only they will be allowed to take out your birth certificate at the county.
you also need two completely ABSURD documents from the US embassy: one is the “declaraci?n de solter?a” (you just sit in front of the consul and swear that you are single, and that’s all that’s required).... then a paper saying that “edicts need not be published in the US when you get married”... VERY IMPORTANT TO SAVE YOURSELF AN EXTRA 8-HOUR QUEUE IN ARTURO SORIA LATER ON: this paper MUST BE SIGNED AND SEALED BY THE CONSUL, as well…
these last two documents must be taken to a place that is ONLY IN MADRID for all of spain, where the consul or ambassador’s signature is LEGALIZADO… to prove that you didn’t forge the documents proving that your not a polygamist (you’re assumed to be trying to take part in polygamy in Spain until you prove yourself innocent). this place is sheer hell, with a line that lasts hours, nowhere to sit, nowhere to go to the bathroom, no food, no drink, no nothing. you may as well be on Ellis Island in 1867. if you leave the line (for instance to pee after 4 hours of waiting), you will be told that you are not allowed to return to your place in the line)
you will need your future wife’s birth certificate and DNI, as well…
take all this to the Registro Civil with the standard form requesting matrimony (not sure where this is in BCN, in Madrid it is in a sheer hellhole on Calle Pradillo, where you will have to wait for hours and hours in the stuffiest, ugliest room you’ve ever seen). you must take a witness with you to suffer the humiliation of the Registro Civil along with the two of you
then you turn all this in and wait for a very long time (a few months) until you are called to pick up the “expediente”, which you then take to the place where you want to get married (a junta municipal, or ayuntamiento, or in the registro civil itself if you would want to get married in a prison-like ambience for some reason)....on marriage day, you need TWO witnesses present and you will be encouraged to rush through the ceremony at light-speed so that the people next in line can start
i was asked for my residency card at several steps in the process, but maybe you can just show your passport instead, i really don’t know. but those are the steps it takes. Spain has fallen into sheer bureaucratic inoperancy and even such a simple, everyday event as marriage has been an utter nightmare to arrange. this bureaucracy is the shame of Spain right now, as couples are forced to wait for OVER A YEAR in some cities to get married, even if both are spanish!!!!
if Spain doesn’t start to fix this mess, the country is going to start to go down the tubes!