esa

Knowing the reasons why you're in which ever group you're in will help. You need to tell then how your conditions affect you and give as much information on that form as possible. Take a look at the descriptors for each group before filling in the form. 
WRAG. https://www.benefitsandwork.co.uk/employment-and-support-allowance/esa-glossary/1354-work-related-activity-group-descriptors

Support Group. https://www.benefitsandwork.co.uk/employment-and-support-allowance/esa-glossary/1353-support-group-descriptors

Reg 29 & 35 here. http://www.cpag.org.uk/content/making-exception

Citizens advice have a guide online for filling out this form too.

If you're still unsure what to do then my advice is to get some help from a disability advice centre near you, CAB or welfare rights maybe.

You do need to make sure that form is returned no later than the date stated on the letter, they won't give extensions for ESA. Send your evidence with the form and don't rely on them contacting anyone for any evidence because it's rare if they do. Expect another face 2 face assessment and if a home assessment is needed then you'll most likely need a letter from your GP stating the reasons why, Good luck.