If you want to add a contact form to your site, then the über-popular Contact Form 7 plugin is a good bet. As can be seen from the little demo below, Contact Form 7 is a very straightforward, real-time validation-enabled form. You can build multiple forms, insert forms to posts and pages, add and remove textfields/textareas etc. In other words, you can easily build a form that suits your needs.
If Contact Form 7 is not to your liking however, then you can of course use any form plugin you’d like. Using Contact Form 7 with Mockup is merely a suggestion. You can find more information about it on the WordPress plugin repository here.