One of the first TODO items is networking the house. These are some relevant links.
Ideas for running cable from the attic along the wall to the right hole in the wall:
1. Make hole. Insert pen laser shooting up. In attic, wave hand around looking for laser or see it on a timber.
2. Use dental floss mirrors or a compact to look up the wall.
Cat5 to Every Room of the House
Wiring your house or office
How to Wire You Home with Network Cable
Wiring Tutorial for 10BaseT Unshielded Twisted Pair
Data Communications Cabling FAQ
The Compass DeRose Guide to Ethernet computer network wiring