Training For An Adopted 4yr plus Dog

I am a new member here. About 1wk ago adopted a 4-yr plus dog (mixed Pom/maltese) to avoid him going to SPCA. Since its not paper/toilet train, now slowly training him. Presently only know how to perform 'shake hand'. Quite aggressive n growled when scolding him, maybe due to past mistreatment/handling by previous owners. Is he too old to be sent for obedience training n if ok what is the cost. Or since he is a matured dog what is the best way to 'self-train' him at home. Appreciate advice from experts here.