Access User Info

Does user_id exist?

  if (get_userdata($user_id) === False)
  {
    //user id does not exist
  }

Get User ID

  if (is_user_logged_in())
    $user_id = get_current_user_id();

Get username

  //For the current user
  $current_user = wp_get_current_user();
  $username = $current_user->user_login;

  //For a different user
  $selected_user = get_userdata($UserId);
  $user_login = $selected_user->user_login;

Get Avatar

HTML ready code to display it
$AvatarHtml = get_avatar($UserId, 32);  //2nd argument is optional size
Avatar URL
$AvatarUrl = get_avatar_url($UserId);