You should always try to delegate tasks in Active Directory based on least-privilege.
For the tasks you wish to delegate, you only need to grant Create Child - User Objects, Create Child - Group Objects and Create-Child Organizational Unit permissions.
To do so, you'll be best off creating an OU immediately under your domain object, creating a group to which to delegate access to, and then grant the above three permissions on the OU using the steps outlined above by Laranostz.
After you have delegated these tasks, you should also make sure that you verify your delegations. For more info how to verify delegations - http://www.activedirsec.com/how_to_verify_delegations.html
There is also a tool that can help verify delegations called "Gold Finger for AD". It was designed by a former Microsoft Security Expert and I believe it is also endorsed by Microsoft.
Disclaimer: I am not affiliated with the vendor of the above mentioned tool. I have used the tool and think highly of it, so I am mentioning it, because it is important to verify delegations.