The aircraft type/model needs to be explicitly certified of IFR operations. Usually, that is done by the TC-holder (manufacturer) during initial certification or later by means of a major change. This means the TC-holder should be able to provide you with this information.
Certification for IFR is much more than just installing some avionic equipment. It regards e.g. flight characteristics, reliability, protection from effects of lightning strikes and HIRF, icing, ...

In addition to the airplane certification, you need to comply with NCO.IDE.A.125 for flight instruments, NCO.IDE.A.190 for communications equipment, and NCO.IDE.A.195 for navigation equipment.

The STC (AFMS) according to which the navigation equipment is installed determines the types of operation you can use it for..

Since NCO.IDE.A.195 requires redundancy in navigation equipment, you need to have 2 navigation units. So at minimum you would need one GPS navigator, and one VOR/ILS.

Some may argue that you do not need RNAV capability below a certain altitude, or unless you are going to follow RNAV routes, but as a practical matter, you do need RNAV capability in today's airspace.

Additionally, I found some facts, that some EASA member countries acting slightly different than EASA would like to comply with.
Check this here for further advice:
https://www.easa.europa.eu/community/topics/ifr-minimum-equipment

Jannes, I don´t say it is not true, but as far as I know there are no IFR specific requirements (paragraphs) mentioned in the common CS. As long as you keep out of icing. Am I wrong?
What about NVFR, does the aircraft also has to be certified for that? Which are the additional requirements (beside instrumentation)?

Pierre, what A/C registration does the A/C has?

Tim-Peter, please have look to the Special Condition for CS-VLA aeroplane to be certified for IFR (VFR Night): https://www.easa.europa.eu/document-library/product-certification-consu…
And here: Special Condition for CS-LSA and VFR Night: https://www.easa.europa.eu/document-library/product-certification-consu…

Both SC provide a good flavor of what needs to be looked at in the context of VFR-Night and IFR certification.
Certainly CS-23 covers all those elements. However, depending on the kind of operations and category, the acceptable Means of Compliance might differ from what is acceptable for IFR ops. Quit obviously an aerobatic aeroplane might have difficulties with the required stability for IFR ops., although CS-23. There is also CS-ACNS to be complied with.

Again, there is a common misunderstanding that simply installing the equipment required by NCO.IDE would make an aeroplane good to go IFR. That is not true. The aeroplane needs to be approved for that kind of operation and the individual serial in conformity with that configuration.

Thank you all for your help.

AC is registered in France and is under NCO and VFR

OK so let's say I have 2 nav means, 2 Com and all equipment required... What do I have to send to the authority ? How much time and how much will it cost me ?

I have heard that the equipment (like for example VOR) used to be send and calibrated so that it can be used in IMC condition... Is that still true ?

Thxs again

Hi Pierre, I recommend you to get in contact with an appropriately qualified maintenance organisation (Part-145 or Part-CAO) to look into your aircraft's configuration and qualification.
I assume you have an EU regulated aeroplane but not a nationally regulated microlight (UL).