How can I restrict salespeople so that they can see only their accounts?

By default, salespeople and sales managers see all accounts and contacts in their organization. However, you can configure the sales application to restrict the view to access based on these conditions:

• The team member is on the account team.
• The team member is the owner or team member of the territory associated with the account.
• The team member is the sales manager (higher in either the resource/territory hierarchy) of those owners/team members.
• For Contacts: The user is a member of the contact team (added manually on a record-by-record basis).

See this article on My Oracle Support for more information: Customer Accounts and Contacts: Restrict 'All' Access for Sales Reps and Sales Managers (Configuration) (Doc ID 2007815.1).

After you finish this configuration, you can use access groups to further modify users' access to records. See How do I create and manage security access groups in CX?.