We have multiple sql server databases on premises and those are setup as clusters, now we have decided to migrate same in RDS, ow how to migrate clusters

Amazon RDS supports Multi-AZ deployments for Microsoft SQL Server by using either SQL Server Database Mirroring (DBM) or Always On Availability Groups (AGs). Details can be found here - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_SQLServerMultiAZ.html

For migrating data from your on-prem MSSQL Server to RDS MSSQL Server, you can use AWS DMS - https://docs.aws.amazon.com/dms/latest/userguide/Welcome.html

Refer to this section specifically for using MSSQL Server as a source - https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Source.SQLServer.html

Refer to this section for using MSSQL Server as a target and that includes RDS MSSQL Server - https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Target.SQLServer.html