What are the steps before I buy a ruin and renovate it?

If you are actually serious about this then you need to spend quite some time viewing peoperties yourself - in person.  The methods of construction of traditional stone built habitation, field stones to make "solid" walls and a roof space vented from outside with sealed cielings do not make obvious modern living spaces with cavity walls, insulation, efficient heating etc.   As for any bank lending money to an amateur property developer with no knoledge or experience buying a something with no resale value should it need to be repossesd - no comment.

@XmoveX,

You need to be careful. The first thing is to check with the town hall to see if there are any restrictions on increasing the construction area on the plot of land you wish. If the work is limited to the existing building on the ground there should be no problem.

Seeking advice from an architect is a good solution, and he may also be able to suggest a contractor. However, there's nothing better than looking for work going on in neighbouring towns, seeing how the contractor works, finding out about him and asking for contacts. There are a lot of fraudsters in the construction industry, and good references are a good way to avoid problems.  Always use contractor who have a building licence. In Portuguese, this is called an "ALVARÁ". Normally when they have this, they put up a number plate in the places where they are doing work with "Alvará nr. xxxx"

You have some support here:

Documents to be aware of when buying a Property in Portugal

https://www.expat.com/forum/viewtopic.php?id=910754

@JohnnyPT Thanks Johnny - this is some really helpful advice. I am not in a hurry and I did already start to look a some plots.....I will collect as much as information as possible and maybe one day - I will find my project

Start saving money for it.  The site you're talking about probably will have a fairly low/negotiable price, and you'll be paying for the rest as you go.  I think Strontium is right about getting any funding.

@XmoveX

Go to your bank first to make sure you can get a loan. If they say yes then you can move on.

@SimCityAT I have an account at the millenium bank and did already asked them for a loan. They were super easy going and just told me, that I need to show the 3 Month of a steady income that is enough to finance the property + 20 % of my own capital

Sounds good to me!