Hello fellow MCOCers (or whatever we call ourselves

)
I only started playing 5 days ago and I think I made a good advancement over this time. I'm almost level 25 with 7 3* champions, and my Gladiator Hulk is nearing his maximum rank.
However, I do have a couple questions.
First, I looked on the internet but couldn't find the answers.
How do I spend my resources wisely?
After I bought a couple of premium crystals for units, I found a guy named Seatin and he said that it's the worst mistake. So now I'm buying those 135-units things for masteries to get my cruelty and precision.
What do I do with arena points? With loyalty points? I don't have any prestige yet, but what's the best way to spend it once I do?
How to use signature stones? Do they even matter?
When it's a class signature stone, I just use it on whoever I can since I don't have too many dupes yet. General stones are used almost randomly.
Ad finally, do I have to rank up my 3* champions or is it better to wait until I start getting 4*?
Thanks in advance!!