The wiki is a good place to start looking around.
A quick summary of the classes:
Rogue- In general high damage low defense. Mostly melee with bard being a support type, ninja and runeblade being the typical high damage melee with ninja being pure swinging pretty much and runeblade enchanting their sword to do extra damage, and ranger obviously being the bow user.
Warrior- In general high defense medium damage. They have paladins and dreadknights which are both super tanky bastards. Dreadknights do magic damage while paladins do healing and party type stuff. They have samurai which are more like armored ninjas with slightly lower damage. They also have dragoons which jump around as their main attraction but are well armored and good at combat.
Caster- In general also high damage low defense like rogue. They have beguiler, wizard and necromancer which all attack in different forms from a range. Beguiler does a bit of extra crowd controlling while necro does pure damage and wizard is a mix. They also have pyromancer which is like a rogue mage which runs around swinging his axe like a madman while throwing fireballs.
Healer- Very mixed here. Healers obviously heal but most are a mix of 2 types. Clerics are the warrior healer with high defense and good healing but minimal offense. Mystics are a caster healer hybrid with root and some damage spells while having great healing, but minimal defense. Disciples are a rogue healer mix with medium to low heals but good damage and utility with their ability to push people around like a jedi. Bloodmage is a weird healer with high health but no armor and below average heals, with some damage spells mixed in alongside some utility like constrict.
My suggestion would be to pick whatever sounds like it would fit your playstyle best. If you like to kill things really fast in melee range then perhaps ninja or runeblade. If you like to kill things from a range, look at the casters or ranger. If you like to help your group look to healers or bard. If you like to be a solo well rounded person look to the warriors.
As for getting started on herocraft I have a simple way of doing things. First, go to warshard, its safer and easier to mine there in peace. You can get there by going through the portal in spawn to the nexus and going through another portal there to warshard. After getting to warshard I would suggest running a solid 700-1000 blocks away from the spawn boat and find yourself some wood to get started. From there start mining with stone -> iron picks and get yourself some items. Make a little hidden base underground with a single hidden chest with your valuables in it. Continue mining until you have enough gold to sell to the DHX to afford an LWC. From there you have to try to sneak by with your gold into the spawn which isn't that hard to do if you play it safe and cautious and sell your gold. Head back to your base and move your valuables closer to spawn and then LWC the chest they are in. From there you can choose to continue leveling crafter by mining or you can choose to level your combat spec by using the items you have already mined to make weapons and go hunt out in the wild.
This is just how I personally would always get started on herocraft. It is probably not the best way or the most efficient but it has worked for me. Good luck out there sir.
Edit: Remember to set your mark in your base. I would also suggest only bringing 1 iron with you to spawn when you go to sell your gold so you can mark there and recall safely with your gold so you don't get killed with it on the way to sell it.
So take 1 iron -> mark at DHX -> run back to base -> collect gold -> recall to mark -> sell gold