You missed that memo from Apple back in June of 2014?

The good news is Apertures still works, at least up through El Capitan. The bad news is nobody knows how long it will continue to work and at some point it will cease to work. It is not supported anymore. So making a plan to move on is a really good idea.

The other bad news is any Versions created in Aperture will not translate to another software. In other words all the Aperture edited Versions must be exported from Aperture as JPEGs or TIFFS in order to preserve the edits.

It sounds like you are using a Managed library. You are going got have to convert that entire library to a referenced library. Additionally you are going to have to export edited versions.

I spite of all the posts that will say "no problem move to XXX software" I am going to say:

(1) Make a plan for exporting and filing/cataloging all the Masters AND all the Aperture Versions. Do this First. Do this ASAP. This must be done while Aperture still works.

After that....

(2) create a test folder. Copy some files, projects, etc into it.
(3) Test out different softwares using the test folder. Find a software you like
(4) Try converting using the test folder.
(5) Convert the Aperture Library slowly don't expect to be able
to do it in one step. It will take time.