Simply insert a de-esser on your vocal channel and sweep through the frequencies in “listen” mode to find what’s causing the problem, then increase the threshold until the sibilance is squashed.
Rather than starting by writing lyrics, try exploring different themes and topics through reading, writing, visual art, or anything else that inspires you. Keep a notebook with you at all times. The more you explore art, the more frequently ideas will come to you, and if you don’t write them down, you’ll likely forget. That way, when you start writing lyrics, you can first look at your list of ideas to see if there’s something interesting to start with.