What is a simple way to transfer HI-8 tapes to my computer?
"HI-8" is an analog format. You will need a hi-8 player or camera with outputs and an analog to digital converter. Consumer grade devices are typically finicky -you get what you pay for. I've used several of the 60-$80 level in the past and had various issues. Such as: not detecting the source, poorly written drivers, very buggy software, low quality capture.
Here's a list of comon some common devices to help your research:

http://vhs-to-dvd-converters-review.toptenreviews.com/

Mostly these come with a software to allow capture and create a digital file. Some also allow you to burn to a DVD / blu-ray. 

A better quality box might be something like this (I haven't tried this unit):
http://www.bhphotovideo.com/c/product/349146-REG/Grass_Valley_602029_ADVC_110_Bidirectional_Media_Converter.html

You will need to get software sperately. Most editing software Premiere Pro, etc. Allows you do capture from a source. You will need to have some knowledge of what codec / wrappers, video aspect; to get a good result.