calcium for plants?

Are you meaning you want to add coral or plants? Most marine plants don't require the addition of calcium. Some macro algaes do but I think regular waterchanges would be enough for most plants.
Most corals on the other hand do require calcium supplements. Large polyp stony corals (LPS) and small polyp stony corals (SPS) both require calcium to grow and will deplete levels in the aquarium quickly. This requires additional calcium/alkalinity supplements. Some easy/common ways to do this is to buy or make a 2 part additive (one part Ca, one part Alk) and supplement as needed. With a 30g tank this will probably be the best method until your tank is more mature and filled with corals (which will make this method expensive to keep up with Ca demand.)
To start off I would recommend starting with some easier soft corals that require little if any calcium demand. Then slowly add the more delicate species such as LPS and SPS which usually requires more pristine water chemistry. Some good beginner corals to look for and research are mushrooms, zoanthoids, button polyps, palythoas, xenia, green star polyps, and leather corals.
Oh and to answer your question, Yes you will need to buy a test kit for Calcium and Alkalinity. I would recommend checking out http://www.drsfostersmith.com. Aquarium Pharmaceuticals makes an OK kit for a reasonable price.
Test kits:
You will want one for Calcium and one for Alkalinity (or KH)
http://www.drsfostersmith.com/produ...atid=15469
2 part additives:
http://www.drsfostersmith.com/produ...catid=4737
A good site to look at different corals:
http://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=597+600
http://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=597+598
http://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=597+599

 

If you're going to go the two part route, just buy it in bulk from bulk reef supply. Their kit makes about 5 gallons of both Ca and Alk for $60. That's much cheaper than buying the premixed stuff Lee linked to.  You're also going to want to keep an eye on your magnesium levels (also included as a supplement in the BRS package)

 

For beginners I would still recommend purchasing a premixed solutions...takes out less variables of things that could go wrong. I've found the DIY 2 part stuff isn't entirely equal, meaning I have to dose different porportions of each... this could be a little more of a hassle for someone just starting out. Also I don't think a 30g tank is going to use 5 gallons of supplement anytime soon.