Anjali, I think you should start with the Keyword Planner from Google. As a free option it is one of the best. SEMRush is great but you need to pay for it to be really effective. After GKP (Google Keyword Planner) I would also take a look at the other, more unknown suggestions Ankit talked about like KWFinder or Keyword Eye – they are quite good and will sometimes reveal keywords you wouldn’t find otherwise (for free).

I recently decided to go with ahrefs after using spyfu for a couple years and trialing secockpit. I was a moz client for awhile too about a year ago. I found spyfu data to be sketchy (or just plain wrong) fairly often, and moz, I don’t know, just didn’t seem like they were really into supporting what I wanted to know. secockpit was achingly slow for a trickle of data. ahrefs isn’t nearly so graph-y as spyfu, but they are so blazing fast and the data is so deep. I enjoy it a great deal, even if it is spendy.

This is an outstanding tool and a must-have for anyone serious about online commerce. Once you learn the way around it is a great suite to add to the collection.  I highly recommend trying it out for your self. With all the features available within the suite, you will start to notice it is the only tool you need to provide the proper information to get organic traffic to your website. Outperform PPC sites by getting the data that will allow you to rank for free instead of paying per click. I have several posts that are on page 1 while others are paying per click to be there. Jaaxy is what got me there.
I use Ahrefs to find ideas for keywords to add into content, and content to create around keyword opportunities. I like how Ahrefs shows keyword difficulty, search volume, traffic potential (how much organic search traffic it’s possible to get when you rank #1 for a parent topic keyword) and lets you group keywords together to create lists. It’s really useful.
It’s important to note that entire websites don’t rank for keywords — pages do. With big brands, we often see the homepage ranking for many keywords, but for most websites this isn’t usually the case. Many websites receive more organic traffic to pages other than the homepage, which is why it’s so important to diversify your website’s pages by optimizing each for uniquely valuable keywords.
We also have a very unique “Local Search” only keyword search that cross references the populations of all towns and cities in USA, Canada & UK. So you can put in a search like “plumber” then choose to see all the cities in “California” with a population of between 50k – 100k and it will spit out plumber suggestions attached to the locale. Pretty neat.