There is much to consider when it comes to getting the best performance from your wireless network.  Firstly let's consider the possible problems that could be hindering your network.

To begin with lets actually confirm it is a problem with your wifi and not your internet in general, to do this plug a network cable direct to your router to either your PC or Laptop computer and run a speed test on your connection.  There are many sites that can do this test for you I would suggest just run a Google search for speed test any first page result will be fine. Run the same test both using the wifi and local cable.

If the connection is bad even when cabled locally
If the result is a horrible internet speed when connected locally then the problem is not your wifi but either your ISP, router or the computer you are using. In this situation the first thing I would do is try using another device on the same internet connection, if the speed is much better using a different device then the fault is with the first device, most likely a software fault as its unlikely both wireless and Ethernet network cards would bother be faulty in the same way. First place to start is with malware and virus clean up then try again.

If the connection is fine when local and bad on wifi at close range
If wifi connection is bad at close range to the router then chances are either the router itself or the wifi adaptor in the client computer is at fault, easiest way to identify this is to bring another client into the mix and see if that PC also has the same wifi problems at close range,  connect a 2nd pc and run the speed test again, if its still bad, then we have a faulty router.

If the connection is fine at close range on wifi but degrades quickly as you move further away.
A standard modern router should be able to deliver reasonable quality signal upto around 40 meters away, if you have a home or this is a workplace where your more than 40 meters from your router then you may want to consider adding wifi access points or signal amplifiers. For most people this wont be the case.

So lets look at possible causes for weak signals over short distance (short being inside a normal size home)

Walls, sounds obvious but walls can cause "some" hindrance to signal strength. Wood and plasterboard and even plastic cause little trouble so the usual indoor walls are usually fine. If you are sharing with a neighbour however brick and concrete provide more of a problem. If this is the case you may want to be smart and place thr router as close to the wall as possible or even better near a window in line of sight of  the sharing home. Glass is also very signal friendly.

Solar panels, people are not usually told in my experience when getting solar panels fitted that it can cause havoc with your wireless devices. I have seen cases where a user has called me due to weak wifi and upon visiting we found his neighbour had recently had solar panels fitted and this killed his network.  In some cases not a lot could be done in other moving the router or wireless amplifiers made the situation tolerable. If this is the case trial and error is best we can suggest, try a few different router locations.

If your still having problems try a router with higher broadcast spectrum, the newer routers use 5Ghz using a router and network card both running on the newest technology can make a massive difference.