Hi there
I would suggest reading ads where ever your posting, see what jumps out at you and why it did. Take notes.
Look at the ads we have from SFI, use some and track them see what makes people click thro.
Then write something that you think would make you want to click and ad. Keep track and thro trial and error you will see what works best. Soon enough you wont think twice about writing ads.
Good Luck