The advantage of dating sites, unlike real life, is that you have the person’s dating profile to give you ideas as to what to bring up in text conversation.
Find something on the person’s dating profile that interests you and ask them a question about it.
It is really that simple.
Not everyone is a good texter. This doesn’t mean that you can’t come up with something to talk about. Just use the person’s dating profile as your guide and the text conversation will evolve naturally.