Increase wireless signal strength of D-Link DSL-2730U WIIF Router

Hi
I am looking to increase signal strength of my D-Link DSL-2730U Router wifi router. It is connect to airtel modem and on my second floor I can hardly use it. Everytime either signals are too weak to connect or are not available at all. I want to use it in my phone and gaming console on second floor but my father use it on first floor so I can not just move it to second floor as his (my father) use internet for office work at home and his mails etc anytime in his phone. So I have decided to increase its signal strenght .. so how can I increase signal strenght.. my budget is limited and I want to save as much as I can . Please suggest me way to increase its wifi signal strength and also I want solution which is easy to follow .. some guide or help ..

thanks.