Look for local classes. Call your community college or continuing education center and ask about learning Castilian Spanish. Smaller colleges may offer only one or two Spanish classes, but you may find that there are options for private tutoring or that a teacher may be willing to help you learn the Castilian Spanish rather than the Spanish spoken in Central and South America.
Look for native speakers. Place an ad in the paper or in an online classifieds website for natives of Spain who can help you learn Castilian. You can offer payment, or exchange English-speaking lessons. Set up a regular meeting time and be faithful to meet and do homework so you stay on track with your learning.
Find online resources. There are both free and paid options for online learning. The BBC offers a free 20-step program for learning Castilian Spanish. Lejos Learning is a paid resource that offers classes online and interaction with the teacher. See Resources below for the links.
Go to Spain. The best way to really learn it is to go where they speak it. Even if you're not a student at a university, you may be able to join a student group going that is going to work, intern or study there for a summer or for a semester. Talk to your local university's foreign language department. You could also plan your own trip. Stay as long as you're able to and do your best to forget that you know English while you are there. Try speaking in Castilian as much as you can. You'll feel awkward at first, but you will quickly catch on to the basics and become more comfortable communicating.