The best place to start is to pick a theme for the room.  This is simple, just look at your interests and find something in that.  If you're an outdoors kind of person then maybe pick a season you like, or a particular type of place you like to go (i.e. mountains).  When we lived in Maine I got really into lighthouses, so now I have a lighthouse hallway.  I'm going to go through the rooms of my house, and then go on to other ideas that you might be interested in. 

Native American is the theme for my computer room/kids toy room.  Its the first room in the house, it would be part of a great room if there wasn't this stupid wall dividing the two rooms.  Anyways, my husband and I really get into Native American things, so what a perfect theme for decorating.  He was in the boy's scouts for a long time, and his troop did a lot of Native American crafts, so finding things for hanging on the walls was not a problem.  He made 4 necklace chokers from bones, so i hung them in a row over the windows.  There are various other things he's made I've got up too.  In addition to that I received 2 beautiful paintings from his parents as a hand-me-down gift.  They were re-decorating, so they just gave me these huge pictures that look awesome in my room.  Then I got something from his mom that is really his brother's.  We rent a house, so we can't paint the walls or anything permanent like that.  I plan on buying some thin cheap leather and making curtains out of, having the bottom fringed maybe.  I have a tall lamp that I'd like to wrap in animal fur or something.  Those are all future projects though.

Fall is my living room.  Well we have a big print of a rainforest, so maybe trees would be a better name for it.  I have taken some fall leaves, from Vermont, and glued them on a mat for a picture frame, and then framed the best leaf of the bunch.  It turned out really nice.  Then I have material to make some canvas slipcovers, and I'm going to stamp either leaves, pine twigs, or something like that on it.  Then the best part of the room is our huge window that over looks the lake we live on.  To tie this room with the Native American room I have a hang up boat with shelves on it, and on the shelves I have a bunch of turtles I've collected through the years (I love turtles).  This room is really still in progress.

Country/Fruits are my kitchen.  When we moved in here, we had grapes stenciled above my sink.  I am not really into the fruits thing, but it follows me, so I figured I'd go with it.  I got these beautiful hand painted apple plates, which are my fancy dishes we never eat off of.  Then I have some towels that attach to the fridge that are fruits.  Then I made one of those plastic bag holders out of denim and then appliquéd apples on to it.  I have blue and white checkered curtains for our huge window that looks out on our lake, same view as the living room.

Lighthouse is the theme for my upstairs bathroom and hallway.  It all started with my husband marking up the stairs in our apartment in Maine with his big black boots.  Once a week I'd have to scrub them off.  A friend pointed out she put some border on the stairs so the border takes the marks and you just remove it when you move.  So I got some lighthouse border paper thinking it was cool since we lived in Maine.  Then a friend of mine whom came to visit took some great pictures (like always) of the local lighthouses.  I framed them and hung them on the way of our dining room at the time, and just added pictures framed in the same frames.  Then we got here, and I got a bunch of stuff for Christmas that was lighthouses.  I got some decorative plates, and model lighthouses that make sounds and light up.  So I moved it all upstairs to our new bathroom.  I got a little trash can and bathroom rug to make it all work.  I have plenty room to add some more pictures as I got, but its really fine how it is too.

Americana is a theme in progress for our master bedroom.  Not much is in there now, but I'm working on a quilt that will be our bed cover.  We have red, white, and blue candles on our window sill.  I plan to take down the light globe from the ceiling and take some glass markers and put some stars and strips on that, that's something cheaply replaced when we move.  I've got Americana picture frames on our dresser with family photos in them.  I plan on hanging some flags on the wall.  This is another in progress room.

Green is the color of everything in our master bathroom.  Just wanted to add this, so you can see that you can just pick a color, or maybe a couple colors, and make it all work.  We used different shades of green but they all kind of work together.  We have green shower curtain, green towels, floor mats, and even a variety of green candles on top of our mirror. 

Kids Rooms are both decorated in their artwork.  I have framed some really good ones, but mostly just cover the walls in all those paintings and stuff we do.  They do things in our playgroup, or when they go to the library.  They are usually so proud of them I'd hate to throw those away so up on the wall they go. 

Ok that's my house, but I am sure your tastes are much different than mine.  So here are some highlights.  Candles add a great little touch and are usually pretty inexpensive.  It's ok to gradually add to the room, its better to have all your rooms have something in it than one done room and the rest of the house empty.  Use the things you already have is a good idea.  You'd be amazed how nice something looks just in a nice frame and inexpensive mat.  A cheap way to get a lot of nice things to frame is buy a photography or paintings book.  Usually they will all be in a theme for you, so just pick some you really like and frame them.  If you can, painting can be a great thing, but make sure it's a color you'd want for awhile.  It's a lot of trouble to paint, and can be expensive.  If in doubt paint an accent wall, live with it for awhile, sometimes a little accent is all you need.  If not then you can easily go back and buy more paint.  Remember that you should take your time and do it right so you don't have to go back and re-do it cause you changed your taste 6 months later.  Decorating is an on going process.