.Unscrew that pretty metal doodad right in the middle of it. It will have two pieces to the metal part. Be ready to catch the glass as it comes off. The bulbs are right inside and available. It is easy if you have a ladder.
The "nipple" at the bottom probably un-screws in order to release the dome section. Alternatively the dome itself unscrews. The lamp might be a "candle" style one if the fitting is small. If you can get a LED lamp, it would be a good idea to avoid the hassle of replacement in the future!
This fixture seems to be tighter than most. I only had to clean mine occasionally, usually dust. Now I live in AZ and have to dust the inside of all my fixtures monthly. During the summer, I have to clean them more because of the moths.
That was a good answer, Becky. We have three of those in our house and my husband changes the bulbs frequently, but it is best a two-person job. Hopefully the new LED bulbs will make changing out bulbs less frequent than conventional bulbs and fluorescent bulbs.
I always get good quality CFL's and rarely need to change them. Phillips is not good quality, I found out. GE seems to last quite a while. I use them exclusively and only need to change one every 3-4 years now.