Do you like bright colours or pastels? A cottage look or something more modern? Are you a minimalist? etc. If you have no idea and just like a mish mash see if you can get hold of a bunch of magazines, they can be really old etc. and cut out pictures of 'things you like', colours, furniture etc. Then try and make a mood board - stick things that seem to go, colours, furniture etc. together and see if that type of theme might fit into one of your rooms. You can always change the feeling of some furniture by adding some stencilling etc. You can decoupage some old shoe boxes/storage with paper you like (wrapping paper can be quite good) and use them as fun storage boxes). Go to your local library and get out some interior design books, even if you don't read them look through them and start deciding what you like and what you don't and then think about what it is you like and what it is you don't. Once you are a bit clearer about your own sense of taste and style then you'll have a better idea of what you might want.
Photos can make somewhere feel really homely and be interesting. You can make a wall of mixed photo frames along one wall with friends and family etc. Making the photo frames all different styles is good as you can do it slowly and therefore budget for it.
Other small things that can brighten up a room: cushions (on a sofa or bed), wall stickers, placemats, a plant, some candles (even if not lit!)
You would be surprised how one or two items can bring a room together. Choose a colour to run through the room and have little bits of it - it doesn't mean that everything matches but does create a theme. You don't have to paint the walls to make them interesting and it can be pricey, also if you move you don't get to take it with you...
Do you have a budget?