I've just been browsing hard-light.net and there seems to be a lot of mods for this game.

Where should I start?

With FreeSpace 1 of course.

First get Freespace 2, either install it via Galaxy or download the installer

Install Knossos:
https://fsnebula.org/knossos/

During the process Knossos will utilize either an existing FS2 installation or the GOG installer to get the needed files for the retail version.

Select your chosen game resolution in the Knossos options after installing, this will now be your launcher and mod downloader for the game.

Now, using Knossos install the Freespace Port and Silent Threat Reborn, if you want the full updated graphics download the MediaVPS and FSport MediaVPS, it will take a while mind you.

The order of play is Freespace, Silent Threat Reborn (don't bother with the retail version, it was a buggy mess with no ending) and Freespace 2.

After that the Blue Planet series is a fan sequel fo Freespace 2, there are a few total conversions like the Wing Commander one and two different Battlestar Galactica ones and several campaing for the classic Freespace and Freespace 2 games.

Bravo! Perfect answer!

I am curios now too. Are those Battlestar Galactica total convertions based on the original series from the 70-80x or on the recent remake from the 00x?

Recent remake.
I've yet to try them but from what I've seen they seem to be pretty nice, at least one of the two (Diaspora) even has cockpits and seemingly lateral thrusters.

Sorry about the Necropost, but it's worth saying: the only downside of the BSG mod, Diaspora, is that it's too short.