untar filename.tr.gz to directory "filename"

You can change directory before extracting with the -C flag, but the directory has to exist already. (If you create file-specific directories, I strongly recommend against calling them foo.tar.gz - the extension implies that it's an archive file but it's actually a directory. That will lead to confusion.)

With Bash and GNU tar:

file=tar123.tar.gz
dir=/myunzip/${file%.tar.gz}
mkdir -p $dir
tar -C $dir -xzf $file

tar -xzvf filename.tar.gz -C destination_directory