You need a software named OCR (optical character recognition) which converts image, pdf etc into text files. Some of the free OCR softwares you can use are:
1. MyMorph which is free but requires registration to use. It can convert apart from TIFF format many other formats like DIF,HTML, MCW, PDF, PPT, PRE, RTF, TTF, TXT, WK1, WK3, WPD, WQ1, GIF, ICO, JPEG, JP2,PIX, PNG, PNM, PPM, PSD, RAS, SUN, SVG, WBMP, XBM etc into text form.
2. FreeOCR: This free OCR software uses the Tesseract OCR engine whose code was developed at HP Labs between 1985 and 1995 and is currently with Google. It is one of the most accurate open source OCR engines and requires Microsoft Net 2.0 framework on windows platform.
Some other freewares are:
1. Simple OCR.
2. Free online converter http://www.newocr.com/
You can google for these softwares and download any of them. Upon installing the software, run it and input your tiff file to get the text output. You can read the instructions for using these software. Also you can try the online website if you feel.