We have a new sink and garbage disposal. Our drain is 18 inches from the floor. What do we do since the drain is the same level as our garbage disposal. What do need to do to get the proper slant. Please advise.

How much does the drain need to be lowered?

You are seeing a common problem with a deeper sink. The bottom of the disposal must be at or above the wall connection otherwise, the disposal will always be filled with water. Where does your dishwasher connect to the plumbing. In some cases, that connection is separate and you may be able to connect there using a "Y". Unfortunately, if you can't do any adjustments to the existing piping, you will need to open the wall and lower the wall connection. If you have PVC or ABS drains, that is within a reasonable DIY job.

If that's two bowl sink with different depth bowls - can you install garbage disposer on the other side of the sink ?

Other than that, you have two options:
1. Do not use garbage disposer
2. Cut open wall and lower the drain inlet

Hope that helps

Milo