Insects and spiders can be fascinating to watch. They are extremely quiet, clean pets that do not need much room to live. Insects and spiders also require little attention or socialization (in fact, they generally do best with little interaction). However, before deciding on an insect or spider as a pet, look at your reasons for wanting this kind of pet. If you are just looking for a pet that is unique or "cool," or for a pet to impress or shock your friends, think again. Insects and spiders are unique and while they are not overly demanding in their care, you must make the same level of commitment to caring for them as any other pet. Too many owners get pets on impulse and once the novelty wears off they tire of caring for them.

Choosing the Right Pet
If you are just starting out with an insect or spider, you are best off choosing a species that is neither delicate nor dangerous. Perhaps the ultimate beginner insect would be the hissing cockroach - they are easy to care for and as a rule are pretty docile and lack serious weapons (not venomous, don't sting, etc.).
Some tarantulas are also acceptable for beginners, such as the Chilean rose, Mexican Redleg, or Costa Rican Zebra. These species are not terribly venomous and are fairly docile compared to some other tarantulas (their venom is akin to bee venom, perhaps, although the venom can provoke an anaphylactic reaction). Stick and leaf insects and mantids are a little more fragile, although they are quite easy to maintain and are quite popular in the UK. Keep in mind that if you have children, most insect pets fall into the "look but don't touch" category, mostly because they are too fragile for all but the most gentle handling (clearly, venomous creatures are also inappropriate for children to handle). Hissing cockroaches are a good exception to this rule, since they are large enough to handle and are pretty sturdy and docile. The traditional ant farm is often interesting for kids too, although kids may lose interest since they cannot interact with the ants, (and an escaped ant colony is a risk!).

What You Need to Consider before Deciding on a Insect or Spider

• Check with local, county and state government agencies regarding legality. There may be regulations in some areas.
• Find out about the care (housing, environment, diet) needed for the pet you are considering. While many insects and spider pets are inexpensive to obtain, providing the right environment can be tricky for some species, especially for the more exotic tarantulas. As a general rule, you want to mimic their natural environment as much as possible. Find out what is needed for food and consider if you can provide the proper diet. For example, hissing cockroaches are easy to feed, whereas stick insects need fresh vegetation and tarantulas need prey insects. Will you be able to provide and and be comfortable feeding the necessary items? 
• Check into the life span. Most insect pets are pretty short lived, but some tarantulas can live for 20-30 years (and females are much longer lived than males).
• Find out how to properly handle the pet. While this obvious for venomous animals, it is very important for all insects and arachnids as they can be injured by improper handling.
• Find out, from a reputable source, the exact species of insect or spider and if it is venomous. Keep in mind that even a mildly venomous tarantula can cause a great deal of pain, and tarantulas have special hairs on their abdomen which they shed if threatened and which can be extremely irritating (and dangerous if they get in the eyes). If you are getting a tarantula, make sure you find out how best to avoid bites, but also how to deal with bites if they should happen, and what to do if exposed to their irritating hairs.

From: http://exoticpets.about.com/cs/insectsspiders/a/insectpider101.htm