There are no prerequisites for starting down one of the Support Skill Trees! You can multiclass if you so choose but you must choose the abilities in the order they are presented on the tree. After you have completed a full Skill Tree, you have the option of choosing one of two Capstone abilities available. Of note, each Capstone ability costs 2 CP.

A Medicus is a support type that specializes in healing both the mind and the body. Their signature abilites, Heal and Apothecary, allow them to restore health and to utilize Poultices that can do a number of beneficial actions, including cleansing negative status effects. In the mind of a Medicus, everyone is going to need a little pick-me-up in these desperate times.
Level 1 ability: Heal
You can spend 30 seconds of appropriate roleplay to call Heal and restore 2 HP to another character or yourself (provided you are not at zero HP).
Level 2 ability: Apothecary
You can now use and craft Poultices. Please see Poultices for more information.
Level 3 ability: Chirurgeon
Your HP is increased by 1 and your Heal ability now only takes 15 seconds. In addition, you can now heal Traumatic Wounds (see Traumatic Wounds for more information)
Battlefield Medicus (2 CP)
Your experience in battles has made you tougher and more perceptive of dangerous threats. You can ignore the first Call made against you in battle. This ability resets each time you pass through the Shadowgate.
Note: this does include ignoring an AOE call if that is the first call against you in battle.
Hospitaller (2 CP)
Once per day, you can channel your Heal ability into multiple people. For one minute, you can use your Heal ability on two people at once, provided you can touch both of them at the same time. In addition, you will start every event with 5 free poultices of your choice.
Note: To effectively use this ability, it is recommended to stay in the backlines and gather many people at once to heal up to 8 people within a minute, AKA setting up somewhat of a field hospital. However, this is just a recommendation. You can use the ability however you desire but it is limited to 1 minute total.
Bard is a support type that specializes in buffing up their allies while bringing down their enemies. Their signature abilities, Inspiration and Taunt, are two sides of that same coin by restoring their allies abilities and tricking their enemies into making bad decisions. In the mind of a Bard, every tune (even the annoying ones) has its uses.
Level 1 ability: Inspiration
You can spend 30 seconds of appropriate roleplay to restore 2 Mana or 2 Prowess to another character.
Level 2 ability: Taunt
You gain the ability to call Taunt. You must spend 30 seconds of appropriate roleplay taunting an enemy before calling Taunt.
Level 3 ability: Rousing Performance
You can spend 2 minutes of appropriate roleplay to bolster your companions before battle. In doing so, anyone (including yourself) who can hear your performance gains 2 temporary HP for their next encounter. This temporary HP will expire at the dawn of the next day if not spent.
Herald (2 CP)
You are the voice of the gods. When you perform for your Rousing Performance ability, those who can hear you can gain 2 temporary Mana OR Prowess points in addition to the 2 temporary HP for their next encounter.
Jack of All Trades (2 CP)
Your base HP is increased by 2. In addition, each of your bardic ability time limitations is reduced by half.