If you can’t really get off your couch or bed, or even get some food, this sounds pretty serious to me. Get to a doctor immediately. Better yet, see a psychiatrist. A good psychiatrist can find the right meds that will help you start to see some light in the world. Right now you’re so deep underwater, you probably don’t even know which way is up.
There are a lot of things you can do on your own that should help, but I don’t know if you can do them from where you are at now. Exercise helps. Eating right and regularly helps. Sleeping regularly for enough time helps (not just hanging out on the couch, but actual sleeping). Hang out with friends. Do anything that will get you out of the house.
If you can get out of the house, and have the energy to do this, find a way to volunteer to help others. That gives you a sense of purpose and something of a feeling like you do matter.
Get therapy. Mindfulness therapy can be very helpful for dealing with the bad feelings you probably have about yourself. It teaches you how to cope with those feelings. You can’t stop them, but you don’t have to let them get you.
Do you live alone? If you have family or friends who are understanding, they can really help a lot—getting you to appointments or work or to take your meds when you have a prescription.
There are don’ts too. A lot of people drink or smoke or use drugs as a way to cope with depression. It is important to get off the self-medications, and onto more effective ones that have side effects that are not as bad. Like addiction.
It is very hard to deal with depression. It almost killed me. That was three years ago. I feel much better now. I did a lot of the things I suggested, although I still have some bad habits. One other thing that helps, and that made a big difference to me, was love. My wife, it turned out, actually loved me, although I was convinced otherwise. In addition, others loved me. Who knew? It was a big help. A big help.
Good luck!