If/when to form a company, as well as which type to form, can be very complex and is unique to each person. Finding a reputable CPA, who specializes in trading, would be the best person to contact. They can learn about your particular situation and recommend the best configuration to protect you.
I used to try to do it all myself. It was agonizing and I never knew for sure if I was doing it correctly. Paying a CPA who specialized in trading really allowed me to focus on what I like... trading!
The Trend is your Friend. ~~