Is your WiFi router in the kitchen? Can it be moved? The kitchen tends to be rough room for WiFi to get out of with cabinets fill with stuff and appliances on all the walls there is just more "stuff" the signal has to go through to get out (unless you have a very open floor plan).
Is your WiFi router stuffed into a corner or underneath something? Again, we want the signal to get out easily ... not always easy because WiFi routers can be ugly boxes.
Does your WiFi router have external antennas? In a perfect world the signal would radiate out from your antenna the same in all directions, but we don't live in a perfect world. The signal from most antennas goes out from the sides well, but not the top and bottom creating a kind of donut pattern. Having the antenna tip pointed toward your computer will result in a low signal. For routers with internal antennas, the same thing can happen if you put the router on it's side.
Most people don't think about it, but radio waves bounce just like light (hey, they are both on the EM spectrum). A carefully placed mirror can actually get the signal around a corner.
Finally, if none of these things work, then you may need to look into adding an access point (another WiFi emitter that is connected to the router with cable) or a repeat/extender (which takes an incoming wireless signal and repeats it wirelessly ... normally placed in a spot in between that gets a decent signal). If you have a smart phone, there are a bunch of free "wifi analyzer" apps that will measure signal strength. With that you can easily walk around and see where you have a good signal and where you don't.
If this is not viable, then powerline networking (which uses your home's powerline to carry a signal) might work. I have seen powerline units work great and I have seen them not work ... it's kind of hit and miss.
And finally, if you have attics you can work with or enjoy patching walls, you can run Ethernet cable from your router to your computer ... the best, but most difficult solution.