...okay. I haven't done this for years, and it was with Slackware, but you'll need a wifi card that will go into "master" mode, hostapd, and probably to set up a bridge interface or do some iptables forwarding tricks.
For the first, run lspci and look for your wireless adapter's chipset number, and then go here:
...and see if there's a "yes" in the AP column. If not, you need a new wifi adapter.
After that, I'd look at this page. It's for Arch, but the basics will be the same for Ubuntu.
From there everything should Just Work(tm).