I’m currently in grade eleven. I’m planning on taking the SATs and SAT IIs this Summer (December/January). When should I start writing the common application? Someone told me to write them the Summer before grade 12, aka this year when I’m doing the SATs, but my question is, how will I know the essay prompts? Do they change every year?
The essay prompts for 2016/2017 have been released. A quick “Common App 2016/2017 Essay Prompts” Google will do the trick. I think it’s a good idea to start now. That’s what I’m trying to do 
Students typically enter US universities in August or September. Aim to submit your application by the preceeding Oct-Dec. Check each school’s website for its deadline. They don’t all have the same deadlines… Allow enough time to take standardized tests like the SAT or ACT.
And to avoid confusion since you’re in the southern hemisphere, avoid using terms like “summer”.
CommonApp applications for current juniors will be live August 1st, however you can look up the essay prompts right now and start working on them.