The German Foreign Minister to the Japense foreign minister(March 1941):
"The tripartite pact had above all had the goal of frightening America, and of keeping it out of the war. America had to be prevented by all means from taking an active part in the war and from making its aid to England too effective. The capture of Singapore would perhaps be most likely to keep America out of the war because USA could scarcely risk sending its fleet into Japanese waters. Roosevelt would be in a very difficult position."
Could someone explain why America would be involved with Singapore and why did Japan not choose to attack Singapore?
I think you're misinterpreting the quote.
The point of the quote was Germany discussing with Japan the best way that Japan could contribute to Germany's war effort against the US. Specifically, Ribbentrop and Matsuoka, the foreign ministers of Germany and Japan specifically, were talking about the function of the Tripartiate Pact between Germany, Italy and Japan. Germany wanted to force Britain to the peace table so that Germany could turn its attention towards the Soviet Union. Ribbentrop thus felt that Japan, which at the moment was only at war with China, could help Germany by attacking Singapore. Fortress Singapore was the symbol of British power in the Pacific, and even more importantly was the gateway to India, the Crown Jewel of the British Empire. Ribbentrop's thinking was that the US would not get involved in the war if Japan attacked Singapore (as opposed to, say, Pearl Harbor) and the fall of Singapore would cause British morale to fall and make negotiation more likely.
Of course, the Japanese had their own problems to worry about. They had a resource issue to worry about, one that required them to conquer the Dutch East Indies. And the Japanese were pretty sure that an assault on the Dutch East Indies (as well as Malaya) would bring the US into the war, so they chose to attack the US as well.
Shirer's Rise and Fall of the Third Reich is where you can read about the Ribbentrop-Matsuoka discussion. There's probably more updated sources by now, though.