You can grab the first line of a file with sed using either the p (print) command or the d (delete) command.
First, the print command can be told to print the line matching the line number 1. That combined with the -n flag, which suppresses all lines not explicitly printed, will print just the first line in the file.
$sed'1 p'README.md# TIL
Second, the delete command can be told to delete all lines that aren't the first (1) line.
$sed'1! d'README.md# TIL
The 1 will match on the first line. By following it with !, that will negate it so that it represents all lines except 1.
See man sed for more details.
Note: there are more efficient ways, not using sed, to get the first line in a file. This is an exercise in using and understanding some sed features.