How to specify which folder is displayed when Outlook starts
To change the folder which be displayed when you start Outlook, do the following:
1. On the File tab, click Options:

2. In the Outlook Options dialog box, select the Advanced tab.
3. Under Outlook start and exit, choose Browse... and select the folder you prefer to be displayed when you start Outlook:

See also this tip in French: Comment spécifier quel dossier est affiché au démarrage d'Outlook.