After create a custom object, admin can simply create a new tab from Setup - Create - Tabs and select Custom Object Tabs then create New button. But, if you are not using Unlimited Edition, there are number of maximum tab we can create based on the Salesforce edition, see the limit here.
Here a tip to create a hidden tab for custom object, meaning admin do not need to create new Tab, but we can use the function, here we go:
1. After custom object created, get the 3 characters Id Prefix
How to get the the prefix?
- If you are using Force.com IDE, look for salesforce.schema for that object
- Using Developer Workbench
2. Type the prefix after Salesforce.com URL
Example: object prefix is a03, type in the URL https://na3.salesforce.com/a03
Change 'na3' with your Salesforce.com instance and make sure NOT to add / after the prefix.