I'm just started this game. How should I set my characters to the triangle form.

Like Teschio said, there's no one, single, obvious, objectively-best formation that everyone has access to. All I can do is recommend a few basic tips on how to choose for yourself. It is less about knowing which crusaders are best and more about knowing how the game works to be able to make judgement calls on your own. I could say "This crusader is best", but if you don't have good gear on that crusader, it might actually make your formation WORSE. That in mind, here's a few general tips...

1) Experiment, all, the, time. As you play, more gear and other such things will wildly change your crusaders' effectiveness. Play around. Explore options. Think outside the box. Even if a crusader/ability doesn't give you a good first impression, give it a try anyway. Remember that gear MASSIVELY affects the game. An ability without gear can be absolute crap, but then when fully geared (with rare/epic/etc) can become the strongest thing available to you.

2) Once a crusader has all of it's abilities bought, stop leveling them up. Tank crusaders typically don't accomplish much on normal, free play idol runs, so don't bother with tanks if that tank doesn't give you much added DPS. The only crusader you should be leveling up is your main DPSer. At least until the end of a run. Always use the "Level All" button right before resetting.

3) There IS a level cap. However, you will not reach it for a very, very long time. Early-game, the most DPS will usually come from earlier (further-left) crusaders since they can reach higher levels for the same price as later crusaders. Until you begin reaching the level cap, it's USUALLY true that earlier DPSers (like "The Washed Up Hermit") will be able to milk out more DPS than later crusaders due to the extra levels. However, keep in mind that once you eveeeentually start hitting the level cap, LATER crusaders become better since they can continue to be leveled up after earlier crusaders have hit their peak.

4) That in mind, while DPS is definitely important, DPS comes from levels, so you still need to be able to afford levels. Your formation should be a...modest...balance between DPS and gold-buffing. Gold-buffers will help keep you able to level up your DPSer, so don't neglect them. Overall, put plenty of DPS-support crusaders in your formation and fill leftover slots with gold finders like Detective Kaine.

5) Expanding on that idea, if you're already able to craft legendary gear, try to get Thalia, the Thunder King's legendary amulet gear. It will add an effect to Storm Rider that permanently (for that run, until you reset) buffs your gold find every time you use it, so it will continually buff both your DPS and your gold find. Plus, it's effects will remain even if you take Thalia out of your formation, so you can just throw him in, use Storm Rider, then replace him with a better crusader until you need to use Storm Rider again. Also, "Magnify" (an ability from Kaine) does work with Storm Rider, so always use Magnify before using Storm Rider. Only the DPS effect gets buffed, not the gold effect, but it's still very useful.

6) ALWAYS participate in events. You likely won't be able to do all the challenges every time, but that's fine. Events cycle around every year, so you won't miss out on anything if you can't do a challenge right now. Try to at LEAST unlock new crusaders, though. A new crusader can potentially give you a MASSIVE overall boost. Event crusaders are usually far, FAR more powerful than the original 20 you start with. For example, there is a crusader called "Mindy the Mime", and she is widely accepted to be the BEST crusader in the entire game by a country mile. Her ability to copy other abilities is utterly unparalleled in it's sheer, universal usefulness in any and all situations.

7) Like Teschio said, when you stop insta-killing enemies and are having even a little difficulty reaching that next 25 levels for your DPSer, use the "Level All" button and reset. Idols are based on how many levels you've purchased. Naturally, that means that if YOU are slowing down, so is your idol gain, so it is better time-wise to reset and start from the beginning where you're back to being fast again.