Firstly, if this question means nothing to you, then do not worry – Simba will take care of it all.
If you are transferring your domain name, then normally things work most smoothly if you transfer the domain name first, and then leave the change of nameservers up to Simba.
Alternatively, if you know that all your settings (websites, mailboxes, domains) are set up on Simba, then you can log in to your existing provider’s website, and use these three name servers:
ns1.simbahosting.co.uk
ns2.simbahosting.co.uk
ns4.simbahosting.co.uk