Software script or writing script? Either way, if you wrote it then the copyright is yours. If you're asking how to best protect your copyright then that's another matter.
It's automatically yours. If you want to make it clear then put the little (c) symbol in with your name
I put a lot of my stories on a Writing site. That gives proof of authorship and date. Sadly the site I used has folded, but there are lots of others. Putting the writing on a Blog site would have the same effect, I think.
Print a copy of your script, put in a nice sturdy envelope; if you have written it on a PC, copy the file onto a CD or DVD and make sure you finalise the disc (so that it is non-rewritable if the media is rewritable) and put that in the envelope also. Seal the envelope, sign across the seal - get a witness to do the same; post the envelope to yourself, preferably by recorded/special delivery, and when it arrives don't open it and instead keep it in a safe place. The postmark will date it, and the signatures will help prove its not been tampered with. If all else fails, the digital media will have the file in a read-only format that will have been dated automatically by your PC. If you want absolute belts and braces, you could also get your witness to write, sign and date a statement saying that they saw you seal the envelope and that they signed it too, before putting the first envelope into a further envelope with the statement also going into the second envelope - and then post it to yourself.
Thank you, Loofah, Palustris and FC for the info, I thought it had to be register though a solicitor etc, if it's that simple i will get on and do it