How can I prepare for next summer?

Let me start by saying - don't assume it's too late.

Much like New Year's resolutions, and contrary to popular belief, you can (and should) start working on initiatives as you realize you're interested. Don't wait until it's the right time of year unless a project or task is truly time-dependent, based on a specific business, weather conditions, or something else. Internships can be like this, but aren't always. And - there are definitely alternate opportunities out there.

Experience is valuable - but especially experience in areas you're interested in and believe in. You can't beat that - and definitely can't beat speaking about them with passion.

What are you interested in? Join a club. Or start one. Find out what people in your area are doing about or with this interest. Go meet them. Go to the library and exhaust their resources. Use the internet. Ask questions. Ask too many questions! You'd be amazed - if you're interested in a topic or area, and want to learn more, the questions keep coming and you later find that they often can lead to opportunities - ones from other people and ones you build for yourself.

For internships, what areas are you interested in? Depending on the field, there may be late summer or part time work available. I always liked having a job while in school, especially if I was learning from it.

I'm not sure if this is the right zone for you to find work or projects, but try this as a resource:
http://www.idealist.org/

Start by identifying your interests and matching them with opportunities offered by your school. I would encourage you to talk to the career center, professors, and family friends (basically anybody in your network) to see if they have opportunities they are aware of or if they can connect you with somebody who may be.

It is great that you are asking this question now so don't be discouraged if you are unable to find anything initially - just keep trying. The more you try, the more doors will open.