You're still young, so you have plenty of time. I think that what you should do now is join a lot of clubs you think you're going to like. Join the ones that really interest you. This will help you find the club you truly enjoy. What Ivy Leagues are looking for is depth not breadth. The reason I'm telling to join clubs is just to get to know your interests and passions. However, once you're a sophomore, or maybe in the middle of freshman year, drop out all the clubs you don't like. Trust me, being in a million clubs does not look half as good to colleges as to having a leadership role or being deeply involved in a few clubs you like. They want to know who you are as a person. Don't make the mistake of being in many clubs. A person in 1 or 2 clubs that has a leadership position, takes time into that club, and is involved constantly WILL get chosen over someone who was a member of 10 clubs.
Running for sca will look good. However, and this is a big however, MOST ivy league applicants are national honor society presidents, student coucil presidents, class officer, blah blah blah. You need to stand out. Think of ways to stand out! Start your own club (specially in something you enjoy doing) or charity, and win awards/honors. Get involed in academic competitions. Doing something unique and useful will definitely give you a shot!
You're taking challenging courses. GOOD. they also like to see that you're challenging yourself. Always do honors, and if you can, try an AP class sophomore year. And then Junior year, because it's the most important year, take as many AP courses as possible.
Tennis is great if you like the sport, and if you're good at it. You don't have to be fantastic, but getting involed will benefit you.
YES SAT and ACT tests are very important. The sooner you begin studying the better. Find an SAT prep book; it will definitely help. Study SAT vocab too. You will need AT LEAST a 2100 on the SAT reasoning test. You're only freshman though, so don't worry too much yet. If you start studying your sophomore year, you'll be fine.
By the way, about the volunteer work... do not do the usual volunteer at an animal shelter and things like that. SO many do things like that. It doesnt make you stand out enough. One of my friends who got into Harvard volunteered at elementary school English As a Sencond Language classes. She stuck to volunteering only in that area. They want to see that you're passionate about something.
Keep your grades up, get involved, take challenging classes, and work hard. Once you're a junior/senior, find a book on how to get into ivy leagues because there is still more you have to do. Letters of recommendation, the admissions essay, and the application itself can make a HUGE difference.
Good luck!