Can I use iphone's Wi-Fi to tether onto computer?

I've heard you can tether iPhone's 3G network on computer but I don't have a data plan, can I use iPhone's Wi-Fi on computer?

If you have wi-fi for your phone, just connect to it with your computer?

You have not been clear with the question

Tethering = use the phone Mobile Data connection to connect to something else
by Cable (old fashioned tethering or by Wi-Fi or even bluetooth (Modern Tethering)

It is not to use the phone to conect to a Wi-Fi netwiork
It is to Make the phone into a Wi-Fi network (name = name of the phone)

if you want a Wi-Fi dongle or card buy one.

for the phone
no data plan, no Tethering
And even if you "have" a data plan, it does not mean Tethering is allowed
Most networks do not allow tethering in the plan cost
- tethering is extra outside the allowance

The only plans that have inclusive Tethering that I know of are
-Three the "one plan"
- T-Mo "Full Monty" plan
- O2 contract plans (not PAYG), but still limited to 1G/2GB per month
All other plans, tethering needs a bolt on or is pay per MB (costly)
The inclusive data is for use only by the phone (though you can still eat data with youtube and iplayer, netflix etc)