You just found out that your pet has cancer. First thing to do is learn what testing is needed, what treatments can be given, and what is the success rate of each treatment option.
There is almost always more than one viable treatment for a pet with cancer. If your vet does not give you a list of options, and the success rate for each, you need to find out for yourself.
A great source of information is the online support groups for pets with cancer, or for pets with the type of cancer your pet has. Of course you want to know what treatment your vet recommends, but it is very important to also listen to people whose pets have had the various treatments.