Include a file to an executable script, grab line 1 and line 2, assign them to data_holders

Include a file to an executable script, grab line 1 and line 2, assign
them to data_holders

Assume that there is a file named a.txt
which contains:
aaa
bbb
i need to create an executable script that
will grab these 2 lines from the a.txt
and then print them in the terminal.
in other words when i run..
./script
it needs to print
aaa:bbb