If your proxy cannot do CONNECT, you lose. Sorry.
Most proxies actually do connect - else how would you visit gmail or anything else doing https - but they only do it for https on port 443.
If your proxy does connect, set the HTTPProxy and HTTPSProxy config options in your torrc -- that will make tor make all requests through that proxy using the http proxy command CONNECT.
See the manual page for more details.
Depending on how restrictive your proxy is, you may also want to set the FascistFirewall option. This will cause it to only do CONNECTs to port 443, which works in most places.
A pluggable transport that transports Tor connection in normal HTTP would also work. But that doesn't exist (sufficiently) yet.