Allowing a computer to connect to wifi from android  already connected to wifi. A unique situation
What you're referring to is called tethering - using the data connection of your phone to share internet connectivity to another device.

Look for tethering options in the settings > Wireless menu or other locations in case your manufacturer has placed it elsewhere.

Failing that, consider using an app like FoxFi or some other tethering app.

Also, a search on Android.SE reveals a number of similar questions - please use the provided functionality. Here's an answered question that might be relevant - Can I use my Android device as Wi-Fi Adapter for my PC (i.e. Tethering to PC through USB, but using Wi-Fi not Phone Network)?