It will sometimes happen that the dedicated plant obsessive will see a plant that he or she absolutely must have, despite not knowing anything about its identity or needs or anything else. And then what do you do? You have to buy it, but what do you do with it once you get it home?
Obviously one of the smarter things one could do would be to buy your plants only from places where the staff knows what they have and how you should take care of them. Not everybody has places like that nearby, though, and even the people who do can't count on them 100% -- we had stuff at the garden center where I used to work that I couldn't identify for people. And if I hadn't tried to grow it at home, and we hadn't had it long in the greenhouse at work, then no, there wasn't a whole lot of useful information I could give about it. (I'm still trying to get an ID for one of the work plants.) So this is not always going to be an option.
Another way to go is to start scouring the internet for information once you get it home. (You are already on the internet, so good for you!) The main catch here is being able to describe your find to a search engine in a way that narrows it down. Obviously searching for "unknown plant" is not going to get you anywhere. As most houseplants come from the same fairly small number of species, though, you could do a search for "common houseplants" or "houseplants with heart-shaped leaves" and possibly get somewhere. Also, forums like those at Dave's Garden (subscription required to post, which limits its usefulness), Garden Web, or the UBC Botanical Garden website are good places to find people to ask. [EDITED TO ADD: CelticRose, in comments, recommends www.cactiguide.com for cactus and succulent questions. I've also been known to refer problematic cactus and succulent questions to Cactus Blog.]
But even if you can't find an ID for your plant, tropical plants sold as houseplants tend to have similar requirements, enough so that you can probably keep yours going for a couple months while you figure out what it is. So here's what I recommend when you don't know what you've got but you know you had to have it.
Default care for plants with thick leaves and stems, especially those with thorns (succulents, cactus):
LIGHT: Filtered sun to full sun. Reduce light if plant develops brown or tan patches on stem or leaves, or if plant becomes red or brown overall. Increase light if new growth is small, pale, or weak-looking.
WATER: If it is not already, make sure the plant is in a sandy, gritty potting mix that will not retain water for very long. Never let the plant stand in its drainage water, or pot it in a pot without drainage: water thoroughly, let it stand for no more than thirty minutes, discard any drainage and put it back. Giving your plant tiny sips of water on a regular basis isn't good either: it won't drown the plant, but it's still not healthy. Don't try to keep this sort of plant constantly dry or constantly wet. Give more water if: the plant's skin wrinkles, it's summer, water runs straight through soil, temperature is consistently very warm. Give less water if: oldest leaves yellow and drop, it's winter, stems become mushy, soil stays damp for long periods after watering. (If stems become mushy, you've probably overdone the water so badly that you'll have to cut off a non-mushy piece and try to root it.)
HUMIDITY: Humidity level is not usually an issue for these kinds of plants, and drier indoor air is more likely to be good than bad. So don't worry about it.
TEMPERATURE: Some of these will discolor if they're too hot or cold, but usually anywhere from 50-90F (10-32C) will be fine.
PESTS: Learn the signs of mealybugs and spider mites, at the very least, and watch for those.
FEEDING: It's probably best if you don't feed, until you know what you've got.
Default care for plants that don't look resemble cactus is trickier, because this is a larger group of plants, and some have very specific needs. But since we're aiming for something that will be okay for most of the plants most of the time, and not perfect care for long-term, this is what I try to do with my unidentified tropicals:
LIGHT: Bright indirect light, filtered sun, morning sun, or something in that general area. Give less light if: leaves bleach, turn tan or gray. More light if: new growth is small, plants with variegated or colorful leaves start to grow solid green leaves, no blooms on flowering plants.
WATER: Most of my plants do fine with being watered only when the soil is dry halfway down the pot. As with cactus and succulents, water thoroughly when you do water, and don't make the plant stand in its drainage water for more time than is absolutely necessary. Also don't use pots without drainage holes if this can be avoided. Water more if: leaves visibly begin to droop or wilt, air is very hot or dry, water runs straight through soil (that last one probably indicates a need for a repotting), older leaves turn brown and drop without turning yellow first, leaves are thin and broad. Water less if: oldest leaves turn bright yellow and drop, air is cold or humid, pot has no drainage, plant receives very little light, leaves are thick or small, or you start seeing lots of small, black, gnatlike insects flying around the plant.
HUMIDITY: Very few non-succulent houseplants will be bothered by moist air, and a few of them actually require it. So it's almost always better to have more humidity than less humidity. Also, although you generally do not want to have a fan or a heat/AC vent blowing directly on the plant, air circulation is almost always better than stagnant air.
TEMPERATURE: Most tropical plants can handle temperatures down to about 50-60F (10-16C), though there are a few with much broader or narrower ranges. If you don't know what you have, you're probably safest keeping the temperature between 65 and 85F (18-29C).
PESTS: Learn the signs of mealybugs and spider mites, at the very least, and watch for those. Scale and fungus gnats are good to know about as well, though scale is less common and fungus gnats are less problematic.
FEEDING: It's probably best not to feed until you know what you have, though it's generally safe to feed a plant half-strength fertilizer at any time when it is in active growth, or a very light sprinkling of a time-release fertilizer like Osmocote. The main exceptions are plants with long, strappy leaves (Dracaena fragrans, Chlorophytum comosum), which tend to be more sensitive to minerals and develop dead, black or brown tips when mineral content in the soil is too high. Those I wouldn't feed at all.
Again, I want to emphasize that the above instructions are meant to be temporary, until you can find out what you've got, and that there are plants out there for which these would be bad instructions. Cryptanthus spp., though spiny-looking and vaguely cactusy, need more water than most cacti, and during the growing season, so do Adenium obesum and Pachypodium spp. Dieffenbachia spp. have broad, thin leaves but will still complain if they're watered too often. Homalomena spp. will suffer if the air temperature is below 70F (21C); Chlorophytum x 'Fire Flash' is best kept out of sun altogether, filtered or not; orchids are an entirely different category of plants and most of these instructions don't apply to them at all; and so on and so forth. But it's better than nothing.