The Belgian government, ending several months of hesitation, decided to begin the immediate deployment of the first of 48 U. S. cruise missiles as scheduled by the North Atlantic Treaty Organization, government sources in Brussels said. The Belgian Cabinet made the decision after several hours of debate, the sources said. They said 16 cruise missiles will be deployed within the next several weeks at an air base near the town of Florennes, 40 miles south of Brussels. The last of the remaining 32 missiles will be deployed by the end of 1987. The Netherlands, which has also hesitated about deploying cruise missiles, is due to make a final decision by Nov. 1.