Oil and grease stains are difficult to remove from porous materials like concrete. There are a few things you can try to get out as much of the oil stain as possible. You can try scrubbing with a stiff, long-handled brush and concentrated detergent. Hose off the area and repeat the process if necessary. Another method is to wet the floor, then sprinkle it with powdered dishwasher detergent. Let it set for a few minutes, then pour boiling water on the area. Scrub the greasy spot and rinse. If you try this, wear rubber gloves to protect your hands. If you prefer, you can generally find various commercial products that are specifically designed for cleaning grease from concrete. Be sure to follow the label instructions carefully. If the product is petroleum-based, you'll need to keep all the doors open for ventilation. Once the floor is clean, you'll want to keep it that way. There are special absorbent products available to catch oil drips before they can soak into the floor, or you can use sawdust. You can also coat your garage floor with a sealant or epoxy paint that will prevent oil stains in the future.