You should look at what you are extending it for, and first, what your current router setup is.

Make sure that your main router is the latest model. Wireless N has the best range at 2.4GHz with 230ft. Routers have two channels usually, 2.4 GHz and 5 GHz. 5 Will be faster, but cannot penetrate solid objects as well as 2.4.

If you're main router is up to snuff, then look at what you are intending it to be extended for. True wireless extension can be hard as there are many factors that can cause interference or confuse your device between basestation signals.

If at all possible, run a CAT 5e/6 cable to the local you need the signal boosted and plug in a "Wireless Access Point". You can snag these at Best Buy for ~$50. The easiest one to setup in my opinion is the "Apple Airport Express" (even if you aren't using Mac's, they still work great)

If you cannot run a cable, get a "Wifi Extender" but beware. Not all work well and they can cause issues. The easiest one I've setup is using an Apple Airport Extreme as my main router and a Apple Airport Express as the extender. They are meant to work together and the setup is a breeze. If you are running Windows machines at home, you will need the download here to configure it: http://support.apple.com/kb/DL1391

Enjoy!