This answer assumes a few things:
- your iMac is already connected to the network (most likely via ethernet)
- that your credentials are all setup (based on your question, they are)
- that you’ve been given the host address for the share drive
To connect to the share drive, you should only have to:
- Make sure the Finder is active
- Press commandK (or, in the top menubar, click on Go and select the Connect to Server… option)
- A Connect to Server window will appear with a Server Address field
- In this field enter the host address (it should start with smb://)
- Click on the Connect button
- You should now be connected and able to access all the files via the Finder
Note: - If you’re not already connected to the network, or you require different credentials for this share drive, then after Step 5 you may be prompted to enter your username and password.
If you do have to enter your username and password, it's likely you will need to enter the username in the form of "DOMAIN\username" where "DOMAIN" is the name of your Active Directory domain. If you're unsure of this, your IT helpdesk should be able to tell you.