You need to own the aircraft to lease it out. In the view and buy page, click the view button next to the plane you want to lease out. If there is no route on that plane you would see a lease aircraft button on the right side of the view page. Click on that, then put in the id and lease rate. Leave id as 0 or blank for open leases on the market. Lease rate is monthly.
oh, and please rtfm.
