There are several things you can do, depending on how much you feel like.
First of all, put the baby in the pram and go to the shop to buy a paper. Every day. Say hello to everyone on the way, even if they look at you like you're an eejit.
Ask your health visitor if there are any mum & baby/ mum & toddler groups in your area, and if there are, go along.
Take baby to the library for the weekly kiddie read session.
Take baby to the baby clinic to be weighed every week - not because you need to get him/her weighed, but to get you out, to speak to people.
Have a look at mumsnet local, or the netmums local section to see if there are other mums in your area begging for someone in a similar situation to get together for a cup of tea or whatever.
Join a club/do an evening course in the evenings when your dp gets home.
Tis completely normal to feel down and isolated at this stage, so don't worry about it, or think you are the only one. We've all been there.
Good luck.
xx