Securing computer before repair

They can't sign into your accounts unless you give them the passwords.

The most cautious approach would be to clone your drive to an external drive then wipe all of your personal data from the Mac before sending it in.

See also:

Get your Mac ready for service - Apple Support

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Re: sensitive information:

I have a somewhat different attitude about this than it seems most folks have. My attitude is that ANYTHING you commit to computer-readable form may become public.

So I do not commit to written form like emails, things I do not want others to hear, and I do not enter my credit card numbers such that they are stored on my computer, EVER. The downside is that when I pay bills, or order online, I sometimes have to type credit card numbers -- typically a few times a month.

Should someone steal my computer, they are not getting my credit card numbers. My credit card numbers are not ON my computer.

The larger issue is that they make no promise to return your files to you. the papers you sign to obtain serve=ise say that you have all the backups you want.