Depending on what you want the file to contain:
touch /path/to/file for an empty filesomecommand > /path/to/file for a file containing the output of some command.
eg: grep --help > randomtext.txt
echo "This is some text" > randomtext.txt
nano /path/to/file or vi /path/to/file (or any other editor emacs,gedit etc)
It either opens the existing one for editing or creates & opens the empty file to enter, if it doesn't exist
Create the file using cat
$ cat > myfile.txt
Now, just type whatever you want in the file:
Hello World!
CTRL-D to save and exit
There are several possible solutions:
touch file
>file
echo -n > file
printf '' > file
The echo version will work only if your version of echo supports the -n switch to suppress newlines. This is a non-standard addition. The other examples will all work in a POSIX shell.
echo '' > file
printf '\n' > file
This is a valid "text file" because it ends in a newline.
"$EDITOR" file
echo 'text' > file
cat > file <<END \
text
END
printf 'text\n' > file
These are equivalent. The $EDITOR command assumes that you have an interactive text editor defined in the EDITOR environment variable and that you interactively enter equivalent text. The cat version presumes a literal newline after the \ and after each other line. Other than that these will all work in a POSIX shell.
Of course there are many other methods of writing and creating files, too.