When you built your static library, you must have had a "myLibraryName.m" file, and you should also have had a "myLibraryName.h" file that the .m file #imports. This .h file should contain the @interface sections for the class(es) defined in the .m file, or at least the public API portions of them, along with any other public definitions (e.g. #defines, enums, structs, etc) that clients of your library should use.
If your original source wasn't organized this way, you should do so now. If there are any private declarations, you can still put those directly in the .m file, so they're not exposed to clients.
After you build your library, you should copy the .a file and the public .h file into a new folder, and add the library to your Xcode project from that folder. Also, in the project build settings, set the Header Search Paths and Library Search Paths setting to that same folder. Using a copy in a separate folder protects your project from failing to build if you start developing incompatible changes to the library.
With these settings, you should be able to #import "myLibraryName.h" in your Xcode project source files, and the public API should be available to the code in those files.
Note that, if your library includes any actual Obj-C classes, you typically need to add the "-ObjC" flag to the Other Linker Flags setting of your project. That's because the default linker behavior is to omit code that isn't explicitly referenced, and because of Obj-C's dynamic nature, this may cause some classes to be left out, causing link errors. With this setting, everything in the library is linked, whether (apparently) referenced or not.