If I somehow manage to mine a block, how can someone with majority hash power decide which transactions to include in my blocks
No one else can influence the contents of the block you are working on.
However, anyone with the majority of mining power would be able to ignore your block, and just work on their own chain instead. Since they have the majority of hashing power, their chain will extend on average more quickly than the rest of the miners combined, thus by ignoring those (and all other miners) they will be able to arbitrarily control transactions. So, it doesn’t matter what transactions you decide to include in your block, because you will ultimately not be part of the longest chain (i.e. most of the work).