Your question is far too broad to answer on YA. We have no idea what interests you, your age or budget so there is no way for us to offer any solid answers as to what YOU can do here.
Any time you visit a new city it is advisable to borrow or purchase a guide book it will give you info on weather, attractions, public transportation, lodging and dining.
Here is My Must Do San Francisco
1. Ride a Cable Car, the only moving historical landmark in the world. Stop off at the Cable Car Barn at Washington and Mason to get a full understanding of this unique form of transportation. Riding the Powell Hyde line will allow you to see Lombard Street the famous crooked street.
2. Visit Alcatraz, this former prison is the most popular destination in San Francisco. The bay cruise to and from the island will give you the opportunity to see the skyline from the water a unique perspective. It is wise to purchase tickets before you arrive as tours are often sold out http://www.alcatrazcruises.com
3. Visit Coit Tower, Beautiful murals inside, impressive bay view from the tower or Pioneer Park at the base. Then descend Telegraph hill via the Filbert Steps on the east side of the park for a unique view of how folks on Telegraph Hill live, more beautiful bay views and the chance to hear and see the famous Parrots of Telegraph Hill.
4. Golden Gate Bridge, not just a feat of engineering, this 1.7 mile span offers incredible views of The City, the bay and the Pacific ocean. Be sure to visit Ft. Point under the bridge for a different view of this San Francisco Icon.
5. Explore Golden Gate Park, this large urban park can consume a whole day. The Japanese Tea Garden, California Academy of Science, the DeYoung Art Muesum, the Arboretum, Bison Paddock, and Paddle boats are all worth a look and watch the sun go down over the Pacific over a drink or dinner at the Beach Chalet.
6. Chinatown, the most densely populated portion of San Francisco offers an abundance of shopping, food and unique architecture found outside of Asia. The best place to buy souvenirs to take home to friends and family.
7. North Beach, traditionally the Italian section of the city which has no beach but abounds in quaint shops, great restaurants and The City’s best coffee options.
8. Civic Center, the largest collection of Beaux Arts buildings in the US. Home to San Francisco City Hall, with a dome larger than the US Capital, the Asian Art Museum and the Modern San Francisco Library with its San Francisco History Room housing an eclectic collection of city artifacts and photographs.
9. Yerba Buena Gardens, this urban park is located near the Museum of Modern Art, the Museum of The African Diaspora, Yerba Buena Center for the Arts, the contemporary Jewish Museum, and and the Cartoon Art Museum, will keep you entertained for hours.
10. Twin Peaks, see The City and the bay spread out at your feet. Especially captivating at night when the lights seem to go on forever.