Sherryl....Pressure wash and use Krud Kutter from Lowe's.
There are a TON of great degreasers out there. Our favorite, to start with is clay kitty litter. It will absorb a lot of oil. (works great on the driveway and garage floor, too). Then the simplest and most accessible thing is dish soap. Almost everyone's go-to is blue Dawn and a splash of vinegar, neither of which should bother the wood. Let that sit on the stain for a bit before you scrub. Depending on how bad the grease stains are, it might require a repeat.
Grease is such a painful thing for us all isn't it. Hopefully these suggestions will help you get rid of those stains on your wooden deck.
Here's a great post on removing grease from wood.