If you didn’t enroll onto a Medicare drug plan (either a stand-alone drug plan or as part of a Medicare Advantage/HMO plan) when you became eligible for Part A and/or Part B and were not covered for creditable drug coverage under an employer plan, then you will incur a penalty when you do join a Medicare drug plan.
Unless you have a special election period (i..e, relocated, loss of employer coverage, etc.), then you will have to wait until Annual Enrollment (October 15-December 7) to select drug plan coverage. That coverage will not be effective until January 1st.
The penalty is 1% of the national average premium (which as an average is @$.30/mo.) for each month you were not on creditable drug coverage. That penalty will either be taken out of your Social Security each month or included in your quarterly billing from Social Security for your Part B.