You increase your actual number of shares by winning them in a contest or by buying them.
You buy them by sending 1 STEEM to @steembasicincome, add another steemian's name in the meme and you both get 1 share.

What most people do is find people who also want to buy shares. You put the steemian's name in the memo and send the money, the other steemian puts your name in the memo and sends the money. Now you both get two shares. You should only do it this way with people you know well.