What kind of bike is it? This type of conversion could cost more than the bike is worth. If you go with integrated shift/brake levers, this could cost several hundred dollars.

The cheapest way to do something like this is to use bar-end shifters and regular brake levers. Tektro V-Brake levers are pretty nice.

If you only want drop bars because you need more hand positions, you could try something like Soma Sparrow bars or moustache bars like the Soma 3-Speed II which both accept the same types of brake levers and shifters that your flat bars already have. There are equivalent bars from other manufacturers, and I mention Soma because those are the bars I am familiar with.

No matter what you do, you will probably need a new stem. If you put drop bars on the same stem as flat bars, you will probably be too stretched out to use them.