proper method to run service drop to detached garage from house 100amp panel

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need to run 3 wire (110, 110 and nuetral and ground wire,approx. 30 feet tapping into or using existing service drop electric co.supplied home with. has 100 amp service currently for home. if necessary will add sub-panel to garage 30 amp or so.want to run from roof to roof attaching to garage as is proper. former owner has run hot and nuetral from single pole 20 amp breaker through 1/2 inch emt with 12 gauge wire. no ground wire,ran circut along side house under eves, then down to ground from back wall of house under- ground for 10 feet then up 6inches of garage wall opening a hole in stucco to enter garage ending in a 2x4 electrical box nailed to a stud. would like to know guage size wire? if i need to use insulators on the garage? if need to install 9 foot copper grounding rod for a sub-panel? (thier is one already used for main 100 amp service panel. Would sincerly appreciate any suggestions or answers to my questions! PLEASE REPLY TO SERVICE DROP FOR DETACHED GARAGE.