You get about 10 points for every $1 spent. You need a minimum of 7500 points to stay in a category 1 or 2 (ex. Fairfield Inn). You can do this using Point Savers which will reduce the points from 10000 to 7500. Remember that there are also promotions this summer that allow you to stay three nights and get the fourth night free. Remember to sign up for promotions.
To get credit for your current stay, you need to either make sure your MR number is part of your "folio" prior to checking out OR you will need to submit a request to the MR support group to credit your account with the nights. I believe you get 30 days to get it sorted out, but sooner is always better. As mentioned above, the point breakdown is fairly well documented with the first free night possible with as few as 7500 points. There are promotions each spring and fall, occasionally more frequent as well, so its a good idea to check your account on the Marriott site fairly regularly to make sure you get signed up for the promotions that fit your travel best. Another tip would be to check out the Marriott Visa card promotions. There are several flavors, however each earns MR points faster when you use them to stay and for most anything. They also have different bonus points and bonus nights associated with them, so it might be worth looking into them. I think the premier card carries 15 bonus nights a year, which makes you a silver member each year you keep the card....so that's a pretty good perk :)