I'm not an interior designer by trade, but i do some work for designers so i do have a few ideas for you.
1) Paint the walls.
i have a dark wood, tongue-in-groove high ceiling in my own family room...which faces North getting little natural light. Two months ago, we painted the walls using Benjamin Moore colors "Rich Cream" and "Desert Tan". The "Desert Tan" wall is our accent wall. LOVE the way they lighten up our dark family room.
2) Window Shades
- if you don't need absolute privacy at night, i'd suggest looking into designer window shades. They have a sleek contemporary look in a variety of textures and patterns. Many companies sell them...such as Hunter Douglas and Graber. (Pull out those metal or plastic miniblinds you have now.)
3) Room Arrangement
- this appears to be a long narrow room so i would create little nooks - one for tv viewing, another for chatting, by simply rearranging the furniture and maybe adding another chair or two.
4) Area Rug
- Add an area rug to the largest nook to cozy it up.
5) Window Treatments
- to add a little more warmth, i would add window treatments - simple, sleek box pleat valances. Use the fabric style, color and texture to emphasize the mood you want to create.
6) Accessories - No need to run out and buy new furniture right away. Use throw pillows in lighter fabrics, colors, textures to lighten up the mood.
A throw can also be added to the couch.
7) Plants - buy some. You don't have to spend a lot. The Walmarts of the world have many delightful plants at reasonable prices.
8) Wall Groupings - Take the little things off the walls and try to create groupings. Again...another area where you don't need to spend a lot. Stores like Michaels, JoAnns sell framed artwork. Looking for interesting ideas? Check magazines and look at what they are hanging - and also HOW they are hanging them.
These are just some ideas. Don't do it all at once. Do it in steps because you may end up changing up and/or refining your process as you go along.
Have fun with it and let it be an expression of who you are, and what you like.