First of all you need to make sure that your carrier allows you to tether. Some carriers do not allow tethering on certain contracts/plans unless you pay extra to use the service, and even then it is often limited to only a couple of gigs.
To be able to tell you step by step I will need to know what mobile you have and what version of Android it's running. =)
you can use the pdanet app (or similar app) or be running a rooted phone with a custom rom like miui.
i have android 2.3 miui and it works perfectly to tether to my laptop. vzw wants you to pay an extra fee for "extra data" and to allow "tethering" when i have unlimited data and the tethering option is included in android.. they just lock it for greed on the stock roms.
i use the phone as a wifi source not over usb but i've heard in certain android versions you can specify to share over usb.
in any case with the limited data plans now... the data would be drawn from the total for the plans so its not like you're freeloading. honestly its all a load of crap and greed.