Learn to read and write two letters and learn to say the related words.
Usually in Indian languages you get four letters and four distinct sounds such as kh, kha, ga and gha. In Tamil there is only one letter for this ka series. And it is sometimes pronounced as ka and sometimes as ga. The context decides the sound.
So instead of learning just one sound, it is better to learn it in words. Listen to the words for ka and cha.
You might have noticed that cha supplements for the sa sound also.
Let’s learn how to write cha and ka. If you learnt to write cha, just a little bit of curve at the end will get you ka. So learn cha first and then learn ka.
Take a newspaper. magazine, any advertisement notices etc. and try to circle these two letters ka and cha. That will help you remember these letters for a long time.
Here is one sample emagazine.