Where you have two individuals at the same household, you will likely want to link these records. The system has a very simple and fast way to do this by householding a record. This article deals with linking records one at a time. You can also household records in bulk. To do this, please see "Matching Households en Mass"
To start, go to one of the records, you'd like to link into a household and click the 'Household' link in the middle section. This will show current householded record, which should be none. From here, you can click the 'Find Household' link.
This will launch a search dialog, where you can search on various fields. If you have an address for your record, it will pre-populate that. However, you can modify the search terms as you wish.
The results of your search are displayed and you can select the correct record and click save.
That's it. If you refresh your screen on the original record, you will now see a household record.