Assuming you have found a converted GTA San Andreas ISO (often labeled "GTA San Andreas Stories" or "GTA SA Remastered" for PSP), follow these steps.
First, you need the PSP emulator. PPSSPP is the gold standard and is free to use. Gta San Andreas Ppsspp Iso File Download
Go to the Google Play Store and buy Grand Theft Auto: San Andreas for $6.99. It includes: Assuming you have found a converted GTA San