Post interesting stuff to your Twitter stream and Facebook page or timeline. (By interesting, I mean things related to your industry, business or products/services that are useful or engaging for current or prospective customers.)
Promote your Twitter and Facebook accounts on your website, business stationery, business cards, etc. and invite people who are interested in your products or services to follow you.
Do NOT make the mistake of buying followers or engaging in any other shady tactics to try to boost your follower counts. Won't do you any good, may actually hurt your brand or even get your account suspended or deleted.