Hi @Madhu J,
If you initially created a AdWords Express account, then you would have to first migrate the account to AdWords.
There are two ways to switch over.
1) You can click the 3 vertical dots in the header at the right hand side and click "view in adwords".
You will be able to access your AdWords Express account in AdWords with the same log in info. You will have some work in front of you to rework your campaigns and put all of the many, many features of AdWords to work. Quality score is really important and it can only be at a good level by utilizing much of the features.
2) Some accounts do not have the dots and cannot be manually migrated. In this case, you have to contact support and they will do it for you. You can call 866-246-6453, Monday - Friday from 9am to 8 pm EST.
If you never set up AdWords Express before, your browser cache needs to be cleared, since the cookie (and history) and auto fill option is taking over. This is the link to AdWords: https://www.google.com/adwords/
Please keep in mind that the two products works quite differently, so migrating an account for the sole purpose of accessing the AdWords Keyword Planner Tool is not advised.
Also, the Keyword Planner Tool now works in conjunction with a campaign, performance, spend activity, etc., so it will not be ideal to use it if you do not have these elements present. If you are simply wanting to do research for actions other than AdWords campaigns, you will find plenty of help with these resources:
These will all provide you with concise & credible material. Each link has many others, so review the menu of each website.
Google AdWords Top Contributor | Google Partner | GYBO | Local Guide | My Profile