Below are some examples of the sign-up shortcode . One thing to note is lists need to be public for someone to subscribe to them. To check that a list is public you can hit the edit link below the list name on the subscribers page.
Full ShortCode with all options:
[sendpress-signup firstname_label='First Name' lastname_label='Last Name' email_label='E-Mail' listids='1' display_firstname='true' display_lastname='false' label_display='false' desc= '' label_width=100 thank_you='Thank you for subscribing!' button_text='Submit']