Typically, to add a CICS transaction, you do the following:

1. Connect a browser to Enterprise Server Administration (http://localhost:86)
2. Edit the server for your CICS region
3. Start the server
4. Go to ES Monitor and Control
5. If your region is still using the demo startup list (DFH$IVP), copy it to a new name so you can change it
6. If your region is still using the demo SIT (also named DFH$IVP), copy it to a new name. Go back to Enterprise Server Administrator and on the CICS tab of the server properties, set the SIT to the new SIT you just created.
7. If you created a new startup list, edit your SIT to specify the new startup list. Now your server should be configured to use a SIT you can edit, which specifies a startup list you can edit.
8. Create a group that will contain your CICS resource definitions, such as your new transaction. Add that group to your startup list. This will let your new transaction to be installed when the region is started.
9. Edit your new group and click the button for a new PCT. Fill in the details for your transaction. The PCT name is the transaction code. The Program name is the name of your COBOL module that the transaction will execute. The other fields can be left with their default values, though you may want to provide a description for your own reference or set other values, depending on your requirements. Click the Add button.
10. Restart your server to pick up the new definition.

When you want to add more transactions in the future, you will only need to add PCTs to your group - the other steps only need to be performed once (unless you want to create different groups for different transactions, etc). You can also install a transaction after creating it, without restarting the server, by clicking the Install button after you add your PCT.

I suggest you go through the CICS Tutorial in the product documentation. It can be found in Micro Focus Enterprise Developer | Programming | Mainframe Support | Tutorials | CICS | Tutorial: CICS.