Call or visit the jobcentre. They will either give you or post a JSA1 claim form and an appointment to go in for a New Claim interview. Make sure the form is filled-in all the way to the back, as you need to claim JSA (Income Based), which is the equivalent of Income Support.
You can stay on it as long as your doctor will give you a sick note. However, after a while you will be called in by the DSS to go through an All-Work Test. If you don't get enough points in this, the DSS will tell you that you must sign on.
You can appeal against the result, though, and still receive IS at a lower rate.