100% agree with Richiev’s points! I’ll add a couple of my own –

1. Writing a logline when you already have a finished script is hard and the feedback you get here (on your logline) will often suggest changes to your story that aren’t necessarily practical if the script is complete. I think most people on here would highly recommend starting with the logline because a bad logline can lead still lead to a great story, but a bad story simply can’t have a great logline.
2. We’re all here to help and we’ve all been there. There probably isn’t any problem that you’ll come across that someone else hasn’t.
3. A lot of people like to hide the most exciting elements for fear that someone is going to steal their idea… my advice is don’t! You can’t get accurate feedback without the readers being able to understand what your story is about. There’s burying the hook accidentally, and burying the hook on purpose – either way the thing that makes your story unique is being lost and you’re never going to get what you came here for.
4. ?Try to refrain from posting a lengthy synopsis of your story. No one is here to write the logline for you (although it frequently happens because it’s fun) but the aim is for you to get to a point where your logline conveys everything that you want WITHOUT needing a full synopsis. Short synopses can be useful, but even then, in my mind the synopsis should be done after the logline. Genre, title and logline – this is where you should start. Nail this and you’ll have something you can refer back to throughout the writing process to keep you on track!
5. It’s a lot easier to critique somebody’s logline than to write your own. The two skills help each other so don’t just post yours and wait for feedback. Read other people’s, and the comments, because this will help you become more familiar with the common problems and give you the skills to recognise issues in your own logline before you post it.
6. Be specific. Ambiguity is where loglines go to die!
7. Every word counts and must be relevant to the story you’re trying to tell!

Some of the points above are things that I’ve learnt the hard way and as Richiev said, you don’t have to take everybody’s advice but remember that everyone here is here to help because we understand the importance and skill involved in a good logline. It’s a really tough thing to be good at, but once you get more comfortable with it, you’ll realise just how useful a skill it can be.

Good luck and I look forward to your first logline!