When you distribute your Hero's skill points the Total Health and troop-type-specific health (eg. Melse Health, Ranged Hwalth) will contribute to troop survivability more than the Offense/Defense skills. Max out Health skills before moving on to Offensive (for attacking setups) or Defensive skills.
The further down the tree you go, the better the bonus a skill provides. Max out Total Helth II before Total Health I, and the same for any other skills with multiple "levels". Start at the bottom and work back toward the top.
As far as forging gear is concerned, initially I'd shoot for Rare or better, for the majority of items, but I wouldn't be afraid to mix a piece or two of Unusual in to a kit. Eventually you'll have so much Legendary gear you'll forget about half of what you have and you'll have hundreds of unused legendary Materials just sitting around collecting dust. Don't worry that, "If I forge this Rare (or even Unusual) gear now I'll never save enough Mats to make it in Legendary later." And bird in the hand is worth more than two in the bush - a 5% Unusual bonus is better than going for three months with zero bonus because I think you HAVE TO have Legendary.
Familiarize yourself with ALL the different types of gear available for your chosen troop type and write them down. Write down the Materials needed to forge each. Write down the different bonuses and the corresponding percentage of each bonus that each piece of kit provides. Learn where each different Material comes from (different rss tiles provide different Mats as bonuses when you clear a tile) and farm the kind of tile that provides the Materials you need. Keep track of the Materials you have at each level. If you know what you have, and you have goals regarding which type of gear you want to build, it makes life a lot easier.
Kill invaders, kill invaders, kill invaders. Aside from the fact that you need to kill invaders in order to get Invader mats to forge ( in many, but not all cases, more powerful Invader gear) killing invaders is also a great way to score points during events. It's okay, in fact it's necessary - if you want the most powerful set of gear possible for a given purpose - to mix Standard, Invader, and the new Special equipment (don't worry about that last type for now).
Definitely look at Gems, as you said you're going to. All attack and defense gear should have Total Health (Ruby), Total Defense (Tourmaline), and your troop-type-specific Attack gems at the highest level you're able to combine them. Again, Health comes first.
