You can create a .a file using the ar utility, like so:
ar crf lib/libHeader.a header.o
lib is a directory that contains all your libraries. it is good practice to organise your code this way and separate the code and the object files. Having everything in one directory generally looks ugly. The above line creates libHeader.a in the directory lib. So, in your current directory, do:
mkdir lib
Then run the above ar command.
When linking all libraries, you can do it like so:
g++ test.o -L./lib -lHeader -o test
The -L flag will get g++ to add the lib/ directory to the path. This way, g++ knows what directory to search when looking for libHeader. -llibHeader flags the specific library to link.
where test.o is created like so:
g++ -c test.cpp -o test.o