So, today i opened some ancients i had saved and ended up getting my first legendary (Rotos)! i also got tayrel and zargala and vergis wich i ve heard are really usefull, any tips for build and usages for those guys? Maybe any of those can help me progress in spider or dragon?

you lucked out big time it seems

Tayrel - he is one of the great epics, he'll go with you everywhere( dragon, spider, FK, etc etc) until you you truly specialize

Speed boots, def% chest and def% gloves, make sure he has enough accuracy to land his debuffs (rule of thumb, dungeon lvlx10; so in Dragon 10 you'd need 100 accuracy)

Zargala - she'll replace your warmaiden, very good aoe def down champ. can also hit hard

Vergis - he is good but I cant say much about him, I think he is more situational

Rotos - incredible champ, mostly used in Arena(I think), best you read up some guides on him. 

Very happy to hear that! also thank for the advices! Wich one do you think i should max out first? im struggling with dragon and spider the most atm (stuck on spiders 9 and dragons 12) 

Don't use any money.

Don't buy anything.

By doing that you keep incentivising Plarium to treat their community like shit.

for Dragon I'd run with, Tayrel, kael , Apothecary, Zargala +1 probably vergis or rotos, depending who works better

Spider is tough, and heavily depends where you are but I think

Tayrel, kael , Apothecary, Zargala, bellower should get you to 11-13 in Spider's den, if you have enough damage

ppl say that until 13? or so its just a matter of nuking the spiderlings, after that you need more elaborate strategies

as to who to max first:

for Arena: rotos

for Pve: tayrel/rotos, probably Tayrel but I have no experience with Rotos, so keep that in mind

My bellower is on a damage build atm, i gave up on the stun ste on him because i was lacking stats (he was most of the time blown up before landing a couple stuns), atm im leveling up my tayrel, he is already 5* and level 40, my rotos is already level 50 but i dont have apropriate gear (im using the gear i was using on my warmaiden), so im assuming i should try to go on and work on zargala and vergis, plus im going to put some effort on armiger, to dragons lair i really have no idea what to do tbh, i guess just getting rotos and tayrel really strong? or maybe using another champion like athel or some other nuker since im lacking on the damage to kill him.

Thanks for your help once again! wish you the best luck.

you should use mostly poison to kill the Dragon, hence kael

make sure to have masteries on your important guys, Warmaster makes a big difference on bosses

Actually need to focus on that for sure, none of my champs have at leats a tier 3 mastery, i spent my gems early on on shards and energy, befor i known i should max my starter champion, so i actually dont have any masteries done, rn the best i can is minotaur 12, should i grind it or should i get a team that can do 15 and grind there?

It sounds like you're trying to do too much at once - which means you will spread your good gear over too many champions. You need to be focused in this game to make progress. Never mind Armiger, focus on progress in Dragon. Never mind making more champions level 50 - make your best ones level 60. 

buy Masteries for Kael , grinding masteries takes forever even if you can farm at 15 

(personally I never contemplated farming masteries below 15, unless it was  some quick tier 1 masteries, to get that extra crit or accuracy early)

then optionally buy masteries for a 2nd champ, one you use everywhere and intend to use for a long time

I imagine its gonna be either tayrel/zargala/rotos

but as L9753 said, focus on getting your most important champs to 60, it will make a world of difference