If there is a first step in choosing an area rug it's probably choosing the size of rug for the space you will be decorating. You need to know what size will look right. Below are a few diagrams, to give you some idea of how an area rug should look in various rooms of your home. Please note that this is not an exhaustive list, just some examples to get you started. I would strongly encourage you to have a look at them and to also have a look at the "Common Area Rug Sizes Charts".
If you have been doing your area rug research, you may have noticed that some say there are certain "rules" that shouldn't be broken when it comes to choosing your area rug size. Others will say that these "rules" are simply guidelines, and shouldn't be set in stone. Personally, I feel it's the later and that it's more about "Balance". Your room should look balanced when complete and your new rug should frame your space.
The #1 Error to avoid when choosing your area rug is.....DON'T CHOOSE A RUG THAT IS TOO SMALL! You don't want your new rug to look like a postage stamp in the room.
For example: Should the legs of the sofa/chairs be fully on the rug, or is it alright to have just the front legs on? Certainly, the rules would dictate that having the rug in front of the legs is a definite faux pas (like the 2nd diagram below), yet I have seen rooms where this is quite acceptable. Again, I believe balance is the operative word.
There is however, one caveat to what I have just said. For a dining room table, my personal preference is to have the chairs fully on the rug when not in use. I just think it looks better to the eye.
TIP: To help you visualize your room better, draw your furniture and floor to scale, then make paper cut outs. This might be time consuming, but it's well worth the effort, and will give you much more confidence when you begin looking for your area rug.
The chart below illustrates the most common Rectangular Area Rug Sizes. Measure your space using the guidelines above, then choose the area rug size that is closest to what you require, and have those dimensions with you when you are shopping for your rug.