First, change your Google account password to something strong (20 characters or more) This can be a phrase like - I_Love_25_Easter_Bunnies Which is easy to remember. Don't use this, it's just an example.
Then, factory reset your phone and sign in with your new google password and enable 2 factor authentication.
Now, on your computer, run a thorough virus/malware scan, let it remove anything it doesn't like.
Next, change all your other passwords and enable 2 factor authentication where possible, using your phone to receive the texts with codes.
Now, the most important part. DO NOT let anyone else have access to your phone.
It might be a good idea to change your number as well.