If the shifters are original friction shifters (no clicks), you will be able to get some thumb shifters to mount on the bars, or some stem shifters to mount on the stem. Generally thumb shifters are more expensive, but I think they are better for two reasons: you can shift without moving your hand at all, and you don't bump them with your knees when standing up off the saddle to climb. Either way, you'll need some shifter cable housing and a clamp-on downtube cable housing stop to go in place of your current downtube shifters.

Edit: I see someone beat me to it! But I'll have to dissent and say I really didn't like the Sunrace thumb shifters (they kept slipping and "ghost shifting" for me), and for a few dollars more you can get some really world-class Suntour thumb shifters such as these (link pulled from ebay after 30 second search).

If you want to have indexed shifting (clicks) you'll probably have to replace the rear derailleur and freewheel and chain, but these can all be gotten cheaply. Then you can put on a Shimano "rapidfire" type trigger shifter on your riser bars. The benefit is convenience and ease of use. The drawback would be this system will much be less reliable and robust than friction shifting.