Hi Aarthi,
If you have got your Skill assessment from TRA which is for Tradesperson, you only need vocational level of English or a band score of at least 5 in all categories in IELTS to claim 15 points.
However, If you are able to score 7 or more in all categories, you will be eligible for 25 points.
I also believe that you are doing your visa process by yourself, if you are doing it yourself it will be a good idea get some help in doing it. Best way to minimise your cost is buying a DIY visa kit.
Some suggestions are:(search in google for "diy australia migration")
http://www.diyaustraliamigration.com
http://www.visacorp.com.au
Hope this helps!