Best way to seal fencing about 12 x6ft panels

I would use Thompson's Water seal if they are wood.

Behr Paints has an excellent sealer for fences, decks, etc. you can find this at HOme Depot and Thompson's Water seal is good but usually this has to be done every few years

here is more info for you

https://www.youtube.com/watch?v=gel9_q0eb34

Here are some good ideas for sealing and water proofing fence panels

https://www.floridastatefence.com/2019/05/30/a-guide-on-how-to-waterproof-a-fence/

It depends on what you want to do but in general, for a wooden fence that you want to just maintain with little change in appearance then go with either Thompson's or another general sealer. Remember that you should clean and reapply annually. If it is new pressure treated wood, you should let it cure 6 months before doing anything with it.

We always used Thompson's.

Buy the best you can afford. Be sure to use exterior sealer. I like Helmsman, but, there are many good brands out there.

https://www.floridastatefence.com/2019/05/30/a-guide-on-how-to-waterproof-a-fence/

You have to put the panels in cement do you mean the spaces because you could use chicken wire

There are 4 x 4 posts that go in between the panels that hold them up when all is together it is all sealed

Is the fence brand new?

What type of wood is it made out of?

Your local home improvement, paint or hardware store will have lots of options for either sealing or staining a wood fence. It depends on if it's been sealed, stained or painted before so your best bet is to bring a photo to one of those stores to discuss with folks there.

Wood Preservative from DIY store for Fencing.

This might be off the mark but when we put up our privacy fence the part of the 4 x 4 that is in the ground was coated with roofing tar.

https://www.youtube.com/watch?v=yd0nDUYHi3U

I personally suggest to use Thompson's Water seal especially for wood.