How can I write and append using echo command to a file

If you have special characters, you can escape them with a backslash to use them as needed:

echo "I am \"Finding\" difficult to write this to file" > file.txt
echo "I can \"write\" without double quotes" >> file.txt

However, you can also use the shell's "EOF" feature with the tee command, which is really nice for writing all sorts of things:

tee -a file.txt <<EOF

I am "Finding" difficult to write this to file
I can "write" without double quotes
EOF

That will write virtually ANY content you want directly to that file, and escape any special characters until you get to the EOF.

*Edited to add the append switch, to prevent overwriting the file:
-a

If you want to have quotes, then you must escape them using the backslash character.

echo "I am \"Finding\" difficult to write this to file" > file.txt echo
echo "I can \"write\" without double quotes" >> file.txt

The same holds true if you i.e. also want to write the \ itself, as it may cause side effects. So you have to use \\

Another option would be to use The `'' instead of quotes.

echo 'I am "Finding" difficult to write this to file' > file.txt echo
echo 'I can "write" without double quotes' >> file.txt

However in this case variable substition doesn't work, so if you want to use variables you have to put them outside.

echo "This is a test to write $PATH in my file" >> file.txt
echo 'This is a test to write '"$PATH"' in my file' >> file.txt