You are going to need to start from scratch as you can’t take a standalone install and turn it into a cluster.
Assuming that you have two new servers for the cluster, build the new cluster. And get everything setup on it. Then move the Exchange mailboxes over to it from the old machine. The users shouldn’t need to do anything other than close and reopen outlook. Outlook will query the original server for the mailbox and it will be told the mailbox has been moved to another server.
If you have to use your existing server, then it’s a little trickier, but still doable.
Setup the machine which is supposed to be the second node and build a single node cluster out of it, and install Exchange Server on it. This will get you a second exchange server that you can then move all the mailboxes over to. After this is completed, and all the users have begun talking to the new server, take the old server out of the environment and rebuild it. You can then add it as a member of the cluster, install Exchange on it and make the cluster redundant.