My girlfriend is American and I am from Spain, what visa should I apply for?

Once you are married you will be an immediate relative of a U.S. Citizen and your wife can file an immigrant visa petition for you so you can enter the U.S. as a lawful permanent resident. This cannot be filed until you are married. The current USCIS processing time is about 6 months. However, after the USCIS approves the petition it is forwarded to the National Visa Center for further processing. After the NVC processes the case it will be forwarded to the appropriate US Embassy or consulate in Spain where you will apply for your visa. The total processing time varies but it will probably take about 12 months from the time your wife files the petition with the USCIS until you have visa in hand.

I have published a step-by-step guide to this process which should be helpful if you and your wife decide the handle the process on your own. Best of luck.