what type of juicer should i buy?

There are 5 main types of juicers, or juice extractors. They each have their own advantages and disadvantages.

Centrifugal Juicers:

Centrifugal juicers are the most common type of juicer that you will find in department stores. They are often upright and cylindrical in shape. They extract juice from fruits and vegetables by grating them into tiny pieces, then using a sieve to “spin” the juice out of the pulp at high speeds (in a similar manner in which a salad spinner extracts the water from washed greens).
Advantages: Centrifugal juicers are much faster when making juice so they are more convenient. They usually have larger “mouths” so you don’t have to cut up fruit as much which means less prep work. They are sometimes easier to clean than other types of juicers. There is a huge variety of centrifugal juicers on the market that can be had on any budget.
Disadvantages: Centrifugal juicers generally do not perform well when juicing leafy greens. If you are serious about making green juices, then look into masticating or triturating juicers. Centrifugal juicers are not as efficient at extracting juice from pulp (resulting in wetter pulp) and they may produce foam in the juice and speed up oxidation.
Examples of centrifugal juicers include the Breville Juice Fountain series and the ever popular (and budget-friendly) Jack Lalanne juicer.

Masticating Juicers:

Masticating (aka “single gear”) juicers use a screw-type auger to grind, crush and “chew” fruits, vegetables and leafy greens. It distributes the juice and extracts the pulp into separate containers.
Advantages: Masticating juicers do a much better job at juicing leafy greens and vegetables than centrifugal juicers, and they produce drier pulp – which means they extract more nutrients. These types of juicers usually last much longer and come with longer warranties – up to 10 years or longer. Masticating juicers can also be used to make nut butters, sauces, baby food, pasta and frozen banana ice cream as well as fruit sorbet.
Disadvantages: Masticating juicers have smaller “mouths” so there is more cutting and chopping of fruits and vegetables when juicing. It takes longer to make juices with these machines than it does in a centrifugal type juicer. Masticating juicers have more parts and take a little longer to clean up after. They cost more money than centrifugal juicers, although high-end centrifugal juicers may cost the same as a mid-range masticating juicer.
Examples of masticating juicers include the Omega 8005/8006 series and the Champion brand juicers.

Triturating Juicers:

Triturating (or twin gear) juicers are high-end juicers and are considered to be the best ones on the market. They work similarly to a masticating juicer but the motor runs slower, which preserves maximum nutrients and promotes efficient juicing. They also have two, interlocking “screws” that grind, crush and “chew” produce in order to extract the juice.
Advantages: Triturating juicers are the best you can get. They are the most efficient juicers and extract the maximum amount of juice (and nutrients), which results in the driest pulp (fewer wasted nutrients). Like masticating juicers, triturating (twin gear) juicers can be used to make nut butters, sauces, baby food, pasta and fruit sorbet.
Disadvantages: Triturating juicers are high end machines so they come with a high end price tag. They are not as quick and easy to use and some force is required to push things like carrots into the gripping, twisting gears.
Examples of triturating juicers include the Green Star brand juice extractors.

Wheatgrass Juicers:

If you want to juice wheatgrass, then you should get a dedicated wheatgrass juicer. Centrifugal juicers are not appropriate for extracting the juice from grasses, and masticating juicers are not the best option either. Wheatgrass juicers are specifically designed to do this job.

Citrus Juicers:

Citrus juicers are specialized to extract juice from citrus fruits like oranges, grapefruit, lemons and limes. Obviously, they are not suitable for making green juices. Citrus fruits can be juiced in centrifugal, masticating and triturating juicers so it is not necessary to get a separate citrus juice extractor if you are getting one of these other units.