Soulstones can be used without consequence - no reason to save them. Likewise, coins can be used without consequence to buy said soulstones. Don't use any of the essences though.

Similarly, there's no harm in awakening any champs whose souls you have, provided you actually plan on using them.

Beyond that - I'd suggest you read some guides, as there is quite a lot behind this system. Here are some suggestions:

https://hellhades.com/raid-shadow-legends-awakening-preview-core-mechanics/

https://hellhades.com/awakening-wishlist-guide/

You can use various means to get soul shards of different values like the summoning shards. The different types will go up in value as the larger ones are harder to get. i tend to open them when i get them. It is all random so you may likely end up with lots of souls for champs you dont have. Worth saving the good ones in case you get the champ in the future. 

Thank you very much. Now I don't see the sense of building up a champion, if it is not worth. 

E.g. I have Temptress with 4* - I could awake her to 4 red stars, IF I ascend her to all 4 basic stats. Would you spend so many potions for her?

Actually I thought to feed her to another good champion... :) 

Your thoughts are correct....most blue champs are food champions and the souls are just...food as well...

Keep on souls you might want to use on champs you stil do not have like perhaps Coldheart...but I normally sell them straight away. 

You will get much more souls you just sell than souls you actually use....but keep the good ones.

Temptress...would even sell a 6* soul as I do not use her...