Tien, Yamcha, Cho'zu, Krillin:
These guys are the beginner trainers. They are the first ones you will probably ever meet when you join the acedemy. They will all help you slightly increase your power, give you some good tips for battles and if you are good enough in thier eyes, they will teach you their special attacks and techniques.
Master Roshi:
This guy is the person of whom it is easiest to get to train you. His methods are a little abstract, but work incredibly well. His techniques are focused mainly on an increase in speed, with slight strength additions too. If you decide to train with him, he will teach you the most basic techniques such as hiding your power level, and sensing that of the opposition. If you're really good then he'll teach you how to do the legendary KameHameHa, but only the best pupils get the opportunity to do this.
Dende:
The main objective of this training is to assist you in learning to meditate and focus on battles, and increase your strength greatly. The most valuable asset that Dende possesses is that on his lookout is the room of spirit and time, where your power can be hugely increased. It isn't really recommended you go here until you have become quite a lot stronger, as with training experience you can improve your power level more significantly. Techniques learnt here include flying, sensing power levels, compressing your own power level and fusion.
King Kai:
The main requirement for this guy is that you must be dead, and have permission to run Snake Way to get to him. If you achieve this criteria, then you may attempt to get the great martial arts master to train you. Whilst on his small planet, you will train under increased gravity, and increase your overall power level massively. If you're really good, you may even be able to learn the elusive Spirit Bomb and Kai-o-ken techniques!
Piccolo, Vegeta:
These two are the second best fighters on the planet. Vegeta with his Super Sayian abilities is slightly better, but also harder to train with. He will only pick those he deams worthy enough, so requesting his training wont help matters, you just have to wait and see if he'll train you. If you are good enough Vegeta will teach you the Renzoku Energy Dan, the Gannet Gun and even the Big Bang Attack. Also, if you are a Sayian and he deems you worthy enough he will help you in acheiving Super Sayian.
As for Piccolo, he is much like Vegeta in his choices, you can't go to him, but he'll go to you. He is not as powerful but will increase you power level incredibly if you train with him. He will teach you how to Hide and sense KI, flight, telepathy and perhaps even the Special Beam Cannon.
Goku:
One of the strongest fighters in the world, the Saiyan is guaranteed to be fluent with insightful tips towards battles, and give you a lot of fighting experience. His methods are unknown as he's never publicly trained anyone, but he does know attacks such as the KameHameHa, Kai-o-ken, and Spirit Bomb, which he'd be likely to teach to anyone competent enough to learn them. He also knows Instant Transmission. Unfortuantly at the moment he can only train you if your dead, because he is too.
Gohan:
This guy is it, the most powerful, living, warrior on the planet. He can teach you a great deal of moves and increase your power level dramaticly. If you are a Sayain he will also help you in acheiving up to Super Sayian Level 3! This guy is even hard to request training from than any of them, and will only train you if you have access to the Room of Spirit and Time...and if Chi-Chi will let him :)
