When a new Lead is created this will also create a new Contact.
To be able to communicate with a Lead in the Web Portal, go to Sales > View Leads > Click on the Contact for the Lead > Select the communication symbol (there will be SMS, phone or Email symbols for the communication methods you have available for the Contacts) > Enter the message > Send.
To communicate with the new lead on the App, go to Sales > Swipe right on the Lead > Select the communication method > enter message > Send.