What will happen to my business when i develop website?

Website can perform better role than your sales and marketing executives. Website provide quick access to your clients and you can target sales online through your website. Website will increase your sales to 200% but you have to optimize your website for Digital Marketing check here

Should i develop only website or something else like SEO, Digital Marketing?

If you develop or already developed website then make sure that you have to plan or planned to target sales through Search Engine Marketing (Paid marketing) or SEO (Organic Google) because without this only website is nothing.