You should provide some more details, but, as far as I know, open putty, insert your droplets' IP number, choose SSH radiobutton and click connect.
Then it will ask for user and password.

If it does not even ask for user/pass, ou may need to check you computer or network if a firewall is blocking you

Oh, first time you connect it throws an warning on you. dont mind it

are you trying to connect with a custom user or root user??

I believe it is simple. just follow the 3 steps process and you are there,

https://www.digitalocean.com/community/articles/how-to-log-into-your-droplet-with-putty-for-windows-users