To catch Mew first go to Cerulean City. Then you go across nugget bridge and go to the grassy area on the left. After go in front of the trainer and pause in sight. Fly, dig, or teleport to Cerulean City. Once you do this you cannot pause. Then go fight youngster and his lv 17 slowpoke(make sure you are a space away from him). If you win you will be able to pause and fly dig or teleport to Cerulean City. Go across nugget bridge and it will pause once you unpause you will find a lv 7 Mew.
To catch Mewtwo you must complete the game and have a master ball. You only get the master ball once. Go to the cave in Cerulean City(surf) and go inside. Go in random directions and if you see a Pokemon it is mewtwo. Talk to it you will have a lv 70 mewtwo. At the begining throw the master ball at him and you will catch him.
Hope this helps