The earliest you’ll start repaying your loan is from the April after you leave your course and only then if your annual income is over the minimum threshold (currently frozen at £21,000 until 2021).
You'll repay 6% of your annual income over £21,000. If your salary is £30,000 per annum, the 6% would apply to £9,000, meaning you'd repay around £45 per month. If your income falls below £21,000 a year, for any reason, repayments will stop until your income rises above the threshold again.
For further details including how much you’ll repay and how interest on the loan is calculated visit Student Loan Repayment.