Can I send a cease and desist letter if my patent is pending? How?

You can send them a notice of your pending patent if it is published and that can, in some specific cases, put them on notice and possibly start to accrue royalty rights. It can cause problems, too, so you should talk privately with a patent attorney to go over the pros and cons.

No - you cannot send a cease and desist letter - since your patent application is still pending. After 18 months of pendency - the USPTO will publish your patent application - unless you asked them not to do so at the time of filing. Once published - you can send a copy of the published application to the "related company" - as a warning that you have a pending case, and under the patent law - you can be entitled to claim damages from them if they are found to infringe granted patent claims that are substantially the same as the published claims - back to the date of your notice letter. Talk to a patent attorney about this for more information. Good Luck!

The law in the U.S. is now "first to file" so you already have an advantage over this other company. If your patent application has been published on the U.S. Patent & Trademark website, they and everyone else are on notice that you are seeking protection for your idea. However, at this point in time, you haven't been granted that protection. There is still a chance that your patent application will be rejected. I would wait until you get word from the USPTO that the patent has been granted.

An application does not give you any present patent rights. Better check and find out whether they also have an application pending. The various scenarios are complex- you need professional advice.