Express as much as you can get out and express every 3 hours (like contractions that's every 3 hours from the start of the last express). Express more often if you can. Express at least 6 times a day, including getting up in the night to express. I used to give myself 4 hours between expresses at night and make up for it by expressing more in the day.
I really really recommend getting a good electric double pump. It will half the time of expressing with a good single pump and being electric it will get on with it no matter how tired you are - with a manual I found in the middle of the night I would be so tired I'd forget to keep pumping and stop for a bit which of course meant I had to be up later. Hopefully the hospital are providing you with a pump but if not I cannot recommend the Ameda Dual Lactaline pump highly enough. Lots of people here recommend their breast pumps but that it based on expressing normally one feed a day. As you will discover, the demand, time and effort is hugely increased for expressing every feed. You can get it from the NCT. I think I got it from
www.simplybreastpumps.com or something like that and it cost around £85. It was my 4th pump, bought when she was 6 months old and I was still expressing for 6 hours a day and I wish I had got it in the first place.
Put one pump on one breast using the opposite hand. Pick up the other pump, put it to the other breast and hold it there with the arm that is crossing in front (because your other hand is at your other breast). This way you have a free hand to change the TV, type on the internet or attempt to read a book.
I would like to assure you that you don't need to breastfeed to maintain a supply. DD never breastfed and I expressed until she was 8 months old - quitting was my Christmas present to myself
For 5 months she was exclusively on expressed milk and would drink over a litre a day. So you can build and maintain a supply just with a pump.
You can freeze everything you get now to use later - the small amounts can be put together to make bigger feeds later on.
Good luck.