Now that your foot is in there, start working on how you can stay there. (And I guess you should figure out how to bring the rest of your body in too. It starts getting cold around November.)
While you're interning, make sure you take the time to socialise and meet people.
Eight Quick (and seemingly obvious) Networking Tips:
Go to events at your agency. During the summer, most agencies are a little slower and do fun things like Summer Fridays (half or full days off) and have lots of socials and events.
Read Drinking Is Good For Your Job
Go to events at other agencies. Be on the look out on LinkedIn, Facebook and with your friends who work at other places to find out what's going on, when and where.
Read What to Do When You're New
Meet other interns. Whether or not they're in your internship program, department or even on your floor -- reach out to them, hang out with them, get to know who they know. You never know -- you may end up being an usher at one of their weddings. (Happened to me, could happen to you if you played your cards right.)
Meet recently-hired people. Talk to other entry-level people and juniors. Find out about how they got in, what they know, who they know and learn all you can from them. Except for bad habits. Don't learn those.
Read Shyness Will Get You Nowhere
Read Small Talking Series 101, 102 - Use the News, 103- 5 Questions, 104 - Introducing Yourself
Use LinkedIn. Update your profile, add all your links, work and relevant experience, join groups and share articles, and reach out to people you meet along the way to keep their contact information and get access to who they know. (BTW I hate when people give me business cards, I lose them and feel like a litterer. I prefer whipping out my phone and adding them on LinkedIn right then and there. It's 2013 after all.)
Read: How to Get Ignored on Linked In
Be yourself. Or be Chuck Norris. Either one of those works.
Here are some related posts with more advice, tips and information
Networking & Following Up Tips to Help People Remember You
Seriously, You Have to Make People Remember You
How to Make Small Talk
People to Always Be Nice To