Trad climbing

I am looking to learn trad climbing, but all courses in Belgium are 18+, so I plan on teaching all theoretical stuff online, and then get experience by climbing With experienced friends, are there any (online, or books...) recommendations for me to learn trad climbing?(I currently know how to climb bolted multipiches)

Buy a bit of gear, go to a rock, and build some anchors on the ground. You're able to learn a lot about techniques and placements this way. There a tons of examples of how to build them online.

Also you'll have better luck in the beginner climbers forum.

Agreed on building some anchors at ground level to practice. I also found good information at the library when I was learning and the videoracle channel on youtube

all good courses of action listed. 

A mentor to evaluate gear placements is the critical aspect .  This will help ensure you are not reinforcing a bad habit that you unknowingly created through unsupervised repetition.

mock leading, following and ground placement all good/safe learning methods

In my first year we would climb every weekend rain or shine. When it rained we’d lead the same easy trad lines on aid. All we had were a few books. Aiding teaches you good placements fast. This was on nuts and hexes in 1978. No reason this approach wouldn’t work today. There were no $300 a day trad classes back then. 

If you are stoked on it you'll teach yourself pretty much everything. Just don't rush and get yourself killed.

And yes 100% you should follow a solid traddie to learn the basics of gear placement, confirm other techniques that you've taught yourself, and ask questions. 

Maybe look for a climbing club near you? You might already have trad climbers in your friend group if you've done multipitch.

These two FREE resources are amazing:

https://rockclimb.videoracles.com/ 

https://www.vdiffclimbing.com/

Books -

Advanced Rock Climbing - Gaines

https://www.amazon.com/Advanced-Rock-Climbing-Mastering-Sport/dp/1493031392 

Rock Climbing Anchors - Donahue / Luebben

https://eastsidesports.com/products/mountaineers-books-rock-climbing-anchrs?variant=33127929020519&country=US¤cy=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gQT=2

Don't both with Freedom of the Hills, there's much better books / web resources out there.

Another good method once you learn basic placements is find a sport route that takes a little gear and try to find a placement a little above a bolt and just use the bolt to back it up. Then just weight it first, then take a little fall, etc. I know another popular method is mock lead climbing. Just being on top rope and placing pieces and practicing just weighting them to build trust and learn. 

John Long, Climbing Anchors.  While the focus is on trad anchors, it also goes over nut and cam placement.  Not sure how difficult it will be to find in Belgium though. 

Good luck.

A lot of the European countries have very strong, alpine clubs and you could become a member and find mentors by doing that. I'm sure they also have clinics, etc.