It's really easy to add a followers gadget and / or a subscribe by email box.
Log into your blog and on the left hand menu select 'layout'.
You'll see that there are various boxes that say 'add a gadget'. You will want to click on one of these, depending on where you would like your followers gadget to appear. I'm going for the side column.
It opens up a window with a list of many different gadgets you could add to your blog. Scroll down until you see one called 'followers'.
Click the little plus icon next to this gadget to add it to your blog. It takes you to another window that looks like this:
! It lets you change all the colours etc. if you really want to though).
Then press save and view your blog. You'll see it's added a followers button.
Now your readers can click the join this site button to follow your blog.
Subscribe by email
You may also want to give your visitors the chance to subscribe by email.
Go back to the layout screen. Click on 'add a gadget' where you would like it to appear. This time scroll through the list until you find follow by email, and click the little plus icon. You can change the title if you want, then press save. That's it, done. Go view your blog and check it out.