For POP accounts you can go to Outlook menu --> Preferences... --> Schedules and edit the Send & Receive All schedule, which is to check every 10 minutes.
I don't suggest lowering this from 10 minutes since many services providers limit your number of connections per hour. The POP protocol was not intended to be an immediate delivery service.
If you're using IMAP accounts then go to Outlook menu --> Preferences... --> Accounts and select your IMAP account. Click the Advanced... button and select the Use IMAP IDLE option. If your IMAP account supports IDLE then you will receive mail as soon as it arrives at your server.
Exchange accounts will check mail once per minute. This schedule cannot be changed.