Figure it out
Make sure you're clear on what you're looking for. What kind of work do you want to do? What type of challenges do you want to face? What type of people do you want to surround yourself with?
Understand that bringing in a foreign intern comes at an additional cost and that you'll have to prove to be able to provide enough value to the company you'll work for to offset that cost.
⤷ Maestro Magurno — Head of Design in San Francisco
Start asking yourself these questions as early as you can and you'll be on your way to figuring out which kind of company you'd want to work for.
Do your reasearch
Once you've figured out your ideal scenario, one way to move forward is to narrow your search down to an industry vertical and start looking for companies that work in that area. Do your research, because not every company offers an internship program.
The size of the company can be a pretty good indicator of whether they are able to accept interns or not (a smaller company tends to have a leaner structure, and therefore might not necessarily have the infrastructure to support an internship program). Of course this is not an absolute rule, so always ask if they are taking on any interns (even if they don't have an internship program in place yet) you could be the first one.
Another great way to get a sense of a company is to talk to people who have either worked or interned there. Ask your classmates, or spend some time on LikedIn to figure out if you know someone who works there.
Start bright and early
Don't wait until the last minute to email companies regarding your internship: there are not only technical times for your visa application you'll need to consider, but it's not unusual for their HR department to take a while to get back to you. Seek contacts early and keep in touch with them, but most importantly, have a portfolio up and running as soon as possible.
Understand that bringing in a foreign intern comes at an additional cost and that you'll have to prove to be able to provide enough value to the company you'll work for to offset that cost. You shouldn't feel limited by this, but be aware that smaller companies are going to most likely be more cost-conscious than larger ones.
Know the territory
As simple as it sounds, you need to know who's who of the industry you want to work in. Learn their work and also their location: is your commute going to be 15 minutes or an hour? Are you going to be close to civilization or completely isolated?
To help you get started, below you'll find a map of advertising agencies, design shops and tech/startup companies in San Francisco.