This is a guide purely to level your pokemon. There will be another topic about how to best train a certain stat.
NOTE that if you use a Lv100 Chansey, this will result in most pokemon leveling from Lv1 to Lv43 -> Not good if you need that pokemon for any of the first 3 Gyms!
Do the following:
Contact another player -> someone you're familiar with or whom you trust.
(Tip: most Gymleaders & PokeHelpers are quite known for their honesty and aren't likely going to scam you)
First ask them if they happen to have a high-leveled Chansey or another pokemon with lots of HP.
(Since exp gaining is based on HP in pixelmon, level doesn't really matter. You could get the same amount of exp from training on a Lv100 Weedle as you get from a Lv40 Chansey)
Make sure this pokemon knows at least 1 move that doesn't inflict damage or lowers stats like attack, sp. attack or accuracy.
(With a Chansey this is quite easy, since he rarely learns such moves).
If they don't happen to have such pokemon, try catching one yourself and level it up by the use of rare candies (highly recommended, since this is the fastest way) or train on another pokemon.
You'll also need a pokemon with super effective move(s) against this pokemon.
Once you have those two pokemon, you trade the high-HP pokemon to your friend (here's where the trust-part comes in).
Start a battle and switch all pokemon you want to level into battle while your friend only uses the high-HP pokemon with the non-damge inflicting move. Use the super effective pokemon as last one. Make the high-HP pokemon faint and you'll see all your pokemon get full exp, not only a share of it as in the Nintendo games.
Keep doing that over & over again, which will result in fast leveling.
Have any more questions? Feel free to post 'em in this topic!