Having difficulties with php artisan serve and composer install

I have created my laravel project and when I type: php artisan serve, I get the following:

**Warning: require(/var/www/vhost/bootstrap/../vendor/autoload.php): failed to open stream: No such file or directory in /var/www/vhost/bootstrap/autoload.php on line 17
Fatal error: require(): Failed opening required '/var/www/vhost/bootstrap/../vendor/autoload.php' (include_path='.:/usr/local/lib/php') in /var/www/vhost/bootstrap/autoload.php on line 17**

I was told on this very site to run composer install in my project dir but I still get the following:

**Loading composer repositories with package information
Installing dependencies (including require-dev)
**Your requirements could not be resolved to an installable set of packages.**
  Problem 1
    - laravel/framework v5.1.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.1.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - Installation request for laravel/framework 5.1.* -> satisfiable by laravel/framework[v5.1.0, v5.1.1, v5.1.10, v5.1.2, v5.1.3, v5.1.4, v5.1.5, v5.1.6, v5.1.7, v5.1.8, v5.1.9].**

Also, I have also checked in my project and the file autoload.php is present. Please I need help on this. am using kali linux

laravel php laravel 5 laravel 4 laravel with laravel tutorial