Ensure your WiFi modem/router is out in the open, located in a central place, and raised as HIGH as you can. It should not be under or behind anything.
Also ensure it's not near fluorescent lights, or located in what is called a "Smart Box". A smart box is the WORST place for a router, (think Faraday Cage).
Many times I helped people who had their modem behind their TV, one of the 3 worst places to place a Wireless transmitter.
The higher you place the Wifi Router, the better. The fewer solid objects the WiFi has to go through, the better the signal, the faster the WiFi. Every solid object the signal goes through reduces the signal level, as does distance. That's why the router needs to be in a central location.