At character creation you get 3 slots for subprofessions. One will be always Forager, and then you can choose others. Every 100 levels you get 1 additional slot for subprofession.
Forager
It is a basic subprofession that is available for everyone. To get it you have to simply make any activity that is related to the Foraging:
Uses pickaxes to get ores from ore veins. There are 6 pickaxes to use and 6 ores to get. Miner works great with Blacksmith occupation because blacksmith can melt ores to ingots to craft equipment ie. iron ore to iron ingot or iron ore and coal to steel ingot.
You can find ore veins in mines and dungeons. The lower you get and the more dangerous place is there, the rarer ore veins you can find.
Be prepared that to get Hellstone you are going to need to face creatures like demons.
Pickaxes can be crafted by Blacksmith. You can also use pickaxes to get small shiny stuff from shiny stone. Try it also on sulphurous stone, vein of ore or gold ore.
To become Miner find Dorgan under Mineroth on the path to Wyrms. This subprofession is available from level 8.
Uses axes to cut trees. There are 6 axes to use and 3 types of trees to cut. Each type has it's own log. Each log can be turned in to wood by using saw. You can also use saw to get wooden ties out of wood of different types.
There are many trees that you can cut. Each tree has different hardness and thickness so you are going to need more hits to cut them.
Wood is needed in woodcrafitng and for the blacksmith to create weapons. To open Woodcrafting menu find big stationary saw around the map. By woodcrafting you can create great house furniture and decorations.
To become Lumberjack find Axell on the top of the Oren Giant Dam (near magic carpet). This subprofession is available from level 8.
Fisherman can catch fish using fishing rod. There are many places that are inhabited in various fish types around Trollheim Online. To go fishing you are going to need baits.
Worms are great for start, but butterflies are much better for catching rarer fish.
There are many different rod that can be crafted by Hunter and many baits that can be attached to them.
Fish can be sold as is or portioned to be used for cooking. Some of them may be useful in alchemy.
As a Fisherman you can also find some fine sunken treasure chests.
To become Fisherman find Willy in the Northport. This subprofession is available from level 50.
Treasure Hunter can open Treasure Chests. There are many places all over Trollheim where Treasure Chest can spawn. There are 10 levels of chests containing unique rewards.
Each one can be opened using lockpicks by more and more experienced Treasure Hunters. But be careful! Watch out for mimics, that may devour chest and wait for any wretch to guzzle - they look identical!
They grow stronger each chest they devour so be prepared for long and exhausting battle. You will be on your own.
The most desirable rares that you can find may be ancient rune emblems. Recognize their origin can empower your runes!
You can read more about rune emblems at What is unique? subsite.
To become Treasure Hunter find Indy in Damak. This subprofession is available from level 50.
Jeweler can craft items. Mainly rings and amulets. The higher the level of Jeweler, the better items it can craft. Jeweler level also responsible for quality, rarity and grade of the items that this subprofession can make.
Jeweler can also use chisel to get cristals out of geodes. Goedes can be found for example in ore veins by miners.
To become Jeweler find Khalil in Nahralis (currently in Oren until the desert update). This subprofession is available from level 100.