I'm little confused so what do I need to do in trust wallet to swap it for safemoon, ?
The swap function in trustwallet is sending a token and receiving another token instead. So, when you swap your USDT tokens, you actually make a transaction and send your tokens to another address.
As long as you don't have any ethereum in the address holding your USDT tokens, it's impossible to move your USDT tokens and therefore you can't swap them.
As already mentioned above, the first thing you need to do is send ethereum to the address holding your USDT tokens.