This genuinely is worth considering as ideas get stolen all the time (paranoid, me??). Sometimes plagiarism is not even committed consciously by the perpetrator. We absorb sentences and word structures and ideas all the time as we move through life, so it’s inevitable that we will reproduce bits and pieces of these when we get creative. If you’re sharing your work with a fellow creative, it should be accepted that you are going to influence them at least a little bit. If they churn out whole chunks of an original work or copy an entire plot without acknowledgement, it is quite different.
First, I’d advise using only people you trust to do your first run of read-throughs and critiques. If they’re your friends, they might not be as tough on your work, slightly biased etc., but it’s better than nothing at all. After that, there are copyright registration places if you live in the UK, but they are pointless in my opinion (we do not have a copyright office here – the US does). They won’t act for you legally if someone steals your work, and will take a small fee to register a copy of your work on the date you submit it. But if you have the raw, dated file stored on your computer, then you can just as easily prove you are the owner of the original work anyway. The law is different between the UK and USA, and elsewhere, so it is worth looking into copyright before you share your work with anyone.