I'd say retake the classes you didn't do so well (d's,f's) in school at community colleges or online (but make sure uc's accept them or whatever school you are applying to). You should check with your high school if they would include the grades in your gpa..not all schools do that. (honestly though..I don't think they would, but it wouldn't hurt to try!)
Preparing for classes over the summer would be a good idea! Talk to the teachers that will be teaching you in the fall, send them emails and buy the textbooks from amazon and take notes over the summer!
your last chance is your first semester senior year! Make it amazing! You must get at least a 3.5 or higher at least..(I'd say shoot for a 4.0), colleges want to see progress(like a trend from freshman year-senior year) in your transcript not just a 4.0 through out your whole high school career. SAT's or ACT's should help too! Take extra AP classes to boost your gpa, but I'd recommend maybe staying away from those if you're studying super hard already (you don't want to stress yourself out..because after all it's your senior year! ) You still have college apps
Make sure you balance out your time and create a schedule.
And don't worry..there's a spot in the application where you can explain yourself for why you received bad grades!
) good luck xx