Been in your shoes several times. You just have to focus on getting yourself comfortable in 'life' again. So start with the basics. Work has been removed from the equation for the moment so that's a non-issue for now. Don't even think about your annual leave or holidays, that's 5 or 6 weeks down the line and not relevant just now.
This 4 weeks you have off is time to not participate in your normal life, but instead to get yourself "well" again, or take steps towards that goal. So that you're better equiped to deal with life when you do go back to it. (And I'm someone who on my first week signed off was halfway down the first isle in Aldi and just couldn't cope with the enormity of it, and had to just put my half-filled basket down in the middle of the floor and walk home).
Focus on you and be kind to yourself. If all you can manage today is get out of bed and brush your hair that's all you need to do. Tomorrow maybe make sure you have a shower and wear fresh clothes. If you can open the curtains that's a bonus. Make sure you're eating well, its all too easy to sit festering and just eat bags of crisps. Opening the front door and sit on your doorstep if you can't face going outside.
I got to the point where I made myself a time table (because structure helps me so much, even now I'm 'better') of one task to do each day. Mondays I would walk into town, buy an apple at the supermarket, and eat it walking home. Tuesdays I would do washing so I always had clean clothes to wear. Wed I would walk to my Grandparents round the corner.
I'm really glad your GP has signed you off for a month, especially if this is your first (and hopefully last!) bout of poor mental health. Sometimes it's only for 2 weeks, and you might have felt obliged to be fixed and back after that length of time. IME you'll need likely a minimum of 6 weeks off. There's a whole lot of difference between feeling ok sitting on your couch in your own home and feeling ok back at your desk/till/phone.
Just be kind to yourself, appreciate the slow-ness of life now that work isn't part of it and take time to get well again. Focus on you, and focus on now. The rest can wait.