No, you don't have to wait. Anyone enrolled in Medicare Part A (hospital insurance) and eligible for Medicare Part B (medical insurance)can choose to receive coverage for both through one of the Medicare Part C Medicare Advantage managed care plans, which are operated by private health insurance providers, instead of directly through Medicare. There is no requirement that you first receive coverage through traditional Medicare Part B for any length of time. The same is true for a prescription drug plan under Medicare Part D.
You should be aware, though, that if you did not sign up for a Medicare Advantage plan within the first six months of being eligible for Medicare Part B, you will have to wait to sign up for a particular Part C plan until that plan has its open enrollment period. Some plans have open enrollment for much of the year, but others only have open enrollment during a limited time in November and December each year.