To create a tarfile containing several files:
tar cvf data.tar *.dat
This creates a tarfile data.tar containing all the files in the current directory which have the filename extension .dat.
Look at the manual page for details of the command options cvf.
To create a tarfile of a directory:
tar cvf project.tar Project Project/README Project/Makefile Project/Makefile Project/main.c Project/screen.c Project/menus.c Project/calculator.c Project/setup/configuration
This creates a tarfile project.tar which contains the directory Project and its contents.
To create a tarfile containing several directories:
tar cvf book.tar text illustrations
This creates a tarfile book.tar containing the directories text and illustrations.