You can create Lookup Field by following the steps given below. Please note that lookup fields cannot be created for Tasks and Events.

1. Log in to CRM with Administrator privileges.
2. Click Setup > Customization > Fields.
3. Select the Module from the drop-down list.
4. In the Field List page, click New Custom Field.
5. In the [Module] Custom Field List page, do the following:
• Select the field type as Lookup.
• Under the Provide Field Details section, specify name of the Lookup field, select the linking module and add the related list view name.
6. Click Save.