How do you put and add mods in 1.13, I don’t know how please help.
I hope your question wasn't meant as "how do I install mods" in general, because that can easily be figured out using Google. Instead, I'll answer specifically for 1.13:
Forge isn't released for 1.13 yet, so there won't be any forge mods anytime soon. I also heard that they're planning a big rewrite, so it will take quite some time before it gets released.
But some other ways of installing mods are available already:
No matter if these downloads say that they're beta, experimental, etc. or not, they should be treated as such, mainly because 1.13 itself isn't even completely functional yet (more information in my other answer).
Some things that also don't work yet:
Update 2019-04-28: Forge is now out for 1.13.2, Optifine and Bukkit are out for 1.13.0, 1.13.1, 1.13.2 and 1.14.0, Sponge and Liteloader are not out for 1.13 yet.