We are using MailChimp for the WP plugin for Newsletter subscribe. You can read more about MailChimp from here
Follow that step below to help you can collect emails of subscribers.
Step 1 - Signup for the Mailchimp account then create your lists to collect email.
Step 2 - Install and active plugin MailChimp for WP and take MailChimp API key
Open https://us1.admin.mailchimp.com/account/api/ and copy Mailchimp API key
Step 3 - Navigate to MailChimp for WP > MailChimp then connect with your Mailchimp account you created from step 1 and insert API key then "Save changes"
Step 4 - Create Mailchimp form to collect email.
Navigate to Forms -> Add new form
Import HTML Code below on section Form mark-up Source
<input type="email" name="EMAIL" placeholder="Enter email address" required />
<button class="button-skew" type="submit" value="subscribe">
<span>Submit</span> <i class="opal-icon-arrow" style="font-size: 75%;">&nbsp;</i>
Step 5 - After creating your plugin. You must set a position where you want to display.
Open Appearance -> Widgets then drag Mailchimp widget to Newsletter Sidebar. You can bring that sidebar to every position you want. ( You can drag to left, a right sidebar for each page. You can drag to Footer position.... )