You can make a "Terms of Service" when you create a thread, and add it there, to which someone who trades with you could agree to, or provide them with a ToS prior to trading with them, it's up to you, but it's not a requirement to have one.
I can't explain it properly to you assuming your native language isn't English it seems. Read The Official Sythe.org Rules and you'll know what rules you'll need to follow.
As this thread is still open I assume you’d still like to complete this task of making a “Terms of service”. You should look over, but not copy other individuals TOS’s here on Sythe to get the basics and as Wortel suggested read Sythe rules. (Especially on plagiarism)
Following what you’ve learned, you should then develop your own TOS with an English speaker who understands your native language as well. This will be better for you in the long run and lead to safety for yourself and those you do business with.