Great advice here, but I'll add a bit more info, even though this is quite old -
There are two sets of nerves that control erectile function that run along the left and right side of the prostate. At a minimum, a prostatectomy disturbs these nerves. Nearly 100% of men have complete ED after surgery and function can improve for up to two year. After that period, it is unlikely function will improve any more. If the surgery included the removal of both these nerve bundles, it is very unlikely a person would ever have natural function again. Only one set is required for an erection, but your chances of natural function returning are lower.
To start, I'd try all the major pills. One may work while other won't. But typically ED caused by prostate surgery is not an issue of blood flow - it is an issue of nerves not functioning properly.
Assuming pills don't work, you have 3 primary options to consider.
1. A Vacuum Pump / Cock Ring Combination. A medical pump that pulls blood into the penis and a ring to hold the blood in place during sex.
2. Injection of a drug (Trimix, for example) You inject the drug directly into the penis and it causes an erection.
3. A penile implant. Either a pump system or a flexible rod.
As others have suggested, there is a weath of information available on the internet, but hopefully this will get you (and others) started.
It would be great if you could report back on how you addressed this!