How can I get wifi on my iPhone 4?

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Okay I really need help here. I can't pay my phone bill in till Thursday, and I usually have wifi at my house so I can use my phone, but I am staying at my grandmother's house (she has no wifi and only 1 desktop computer, what I'm using now) in till New Years Eve (This Tuesday) I REALLY need to use my phone to get in contact with a friend for very important reasons (I also can't use any social media on this computer so no Facebook). I also need to do a lot of other things...Is there any possible way I can like download an app or set up a wifi thing on her desktop so I can get my phone to have wifi on it? Thanks for the answers in advanced, I appreciate it! (: Sorry if this sounds like a stupid question lol