You can set the
lang attribute in your
<html> tag if you want to. The w3c recommends setting it.
If you do set it, you the value should just be
en_US. It is a language identifier, not a locale identifier. It should only specify the language, and not the language and country.
However, setting the
lang doesn't actually do anything. That attribute is not used in any way as far as I know. Google says that they ignore it because they find it more reliable to detect the language from the words used rather than rely on meta-data.
hreflang tags to target alternate versions of your page to different languages. See https://support.google.com/webmasters/answer/189077?hl=en However, if you only have one language, you shouldn't use hreflang tags.
If you do end up targeting your site to only the US, Google will not send traffic from other countries to your site. If the traffic from other countries is valuable to you, you should not try to target your site to the US only.