Two questions, answered both ways round. Put in a monthly amount, a term and an interest rate to see what you end up with, or name a target amount and a deadline to see what you need to save each month to get there.
A common rule of thumb is 20% of take-home pay towards saving and debt repayment, but the useful number is the one you can sustain. Consistency over years matters far more than the rate in any single month.
Contributions are applied at the end of each month, after that month's interest. This is the conservative assumption and matches how most savings accounts credit deposits.