Hello tigercat13. Countertop overhangs should not exceed 12" without additional support, so yes - if it overhangs 18" you DO need some support.
Try metal shelf brackets in a style that suits your decor & be sure to space them out accordingly. Your countertop fabricator will know the measurement.

Best of luck to you!
~Lynn

If there's crown above the cabinets, match the corbels to the molding. Definitely need support - fast.

You can have the granite fabricator install steel supports that will not even show unless you look under the counter. I use these all the time even when I have legs on each end of a long island.

Steel supports that can be hidden or go with full wood corbels. If you want to buy them and do it yourself, check with Osbornewood.com - they have nice stuff.

What Durham says, or you can use an aluminum underlayment instead of plywood.