> If I somehow was able to mine a block, How can someone with majority hashing power can decide transactions to include in my block

Nobody else can influence the contents of the block you are working on. 

However, someone with a majority of the mining power will be able to ignore your block, and just work on their own chain instead. Since they have the majority of the hash power, their chain will on average be extended more quickly than the rest of the miners combined, and thus by ignoring your (and all other miners') blocks they will be able to arbitrarily censor transactions. So, it won't matter what transactions you decide to include in your block, since you block will eventually not be a part of the longest (i.e. most work) chain.