Can I share posts from my facebook page instead of creating new post?
Sure you can, you need to give us more details though, share them where? 
Ok what I need to do is make a campaign for my facebook page + twitter wall (1 campaign).
After that I have to make 2nd campaign for facebook that will share the already posted posts from 1st campaign to the groups that I have joined.
This way I will promote my website + my facebook page, won’t I?
Sorry if this seems very retarded for most of you, but I am 5 days noob and I am trying to build my 1st campaign.
Also I have 1, a little side, offtopic, question.
I am using wordpress with some kind of e-commerce version (but not exactly) plugin and I have products. Is it possible to automatically share this products instead of building the posts for my campaigns 1 by 1, maybe with RSS feeds? If so, how can I do this for products? Because I thought I ca do that only with posts…
Edit: found the option where to share from a specific page
Yes, you are right 
Yes, you can totally use RSS feed to scrap posts from your website, you can read more about it here: Add posts from RSS
I’m glad you have figured almost everything out  Don’t worry you will be an expert in no time if you kept going like this.
 Don’t worry you will be an expert in no time if you kept going like this.