How to be ready for The Rise of Skywalker?

TL;DR

Just watch the original and sequel trilogy and you'll be fine.

Long version:

It'd be nice if you watch the last 8 episodes and have deep knowledge about Star Wars characters to understand all of the easter eggs and references in The Rise of Skywalker. However, this shouldn't be necessary.

This movie is a direct sequel of The Last Jedi and the third installment in the sequel trilogy, so you have to watch the last two episodes. Now this sequel trilogy takes place after the events of the original trilogy and the latest movie shows the return of

Emperor Palpatine,

so you have to watch this trilogy as well. You can skip prequel trilogy and animated shows if you just want to get to know the characters and a few easter eggs in Episode IX.

From Screenrant (emphasis mine),

In mentioning The Last Jedi, the next film up has to be The Force Awakens. This is the movie that kicked off the trilogy and was also directed by Abrams. Any longterm plans he secretly had for the Star Wars franchise that The Rise of Skywalker could address may very well be rooted in what The Force Awakens does. On a similar note, the return of P******** is significant for not only The Rise of Skywalker but the entire franchise. Since he is not seen in the prior two saga entries, knowing what happened to him last would be beneficial. For that reason, seeing Return of the Jedi will go a long way in establishing who P******** is and what he wants.

While these are the most important Star Wars films to see before checking out The Rise of Skywalker, there's sure to be Easter eggs to more movies than just these. Knowledge of the prequels, other original trilogy entries, The Mandalorian, and even animated shows like Star Wars: The Clone Wars and Star Wars: Rebels would be nice, but they aren't necessarily required viewing. So, if you're looking for an abbreviated refresher of the galaxy far, far away before seeing Star Wars: The Rise of Skywalker in theaters, the last three "Episodes" would be the best way to go, along with The Mandalorian episode 7, which contains a small connection to the new film.

However Anthony Daniels, the actor who played C3PO says,

There are only two months to go until Star Wars Episode IX: The Rise of Skywalker hits cinemas. The circle is now complete,” the man behind C-3PO stated to SFX magazine, according to “comicbook.com. “The saga is almost over and the story just about told.”

Daniels urged fans to go ahead and watch all of the eight other movies in the saga. He even specified that you should watch “an episode a week to get ready,” and start with Star Wars Episode I: The Phantom Menace. So, going in the proper order according to the timeline: prequels, originals, then the sequels.

Or you can watch all episodes including Rogue One in chronological order as mentioned here.

There is an appearance of Lando Calrissian, a character from Star Wars Episode V and VI and Solo: A Star Wars Story. However, there doesn't seem to be need of watching Solo: A Star Wars Story, just episode V and VI.

I recommend watching all of the previous The Skywalker Saga films: The Phantom Menace, Attack of the Clones, Revenge of the Sith, A New Hope, The Empire Strikes Back, Return of the Jedi, The Force Awakens, and The Last Jedi.

You DON'T need anything from any of the TV series (Clone Wars, Rebels, The Mandalorian, Resistance) or the other stand-alone films (Rogue One, Solo) to understand the new story, and while the original trilogy (ANH, TESB, ROTJ) is mostly the trilogy that is often referenced in the new trilogy, there are subtle references, especially with its two leads: Rey & Kylo Ren, that dance around the whole journey of Annakin Skywalker and the failures of The Jedi in the prequel era.

The Last Jedi in particular calls 'the past' into question, as some events nearly repeat!

_{For more details on the relevance of the prequels, see my answer here.}

Along with the fact that,

The biggest villain of the piece, in some way, factors into the new film, as he was previously the catalyst for everything that has happened in both previous trilogies and is the actual reason why there is even a "Skywalker Saga" to begin with! Even the title of the first episode, The Phantom Menace, actually mostly refers to THIS character!

_{However, if you find that you like the story of the sequel trilogy, you may also find yourself interested in the new TV series The Mandalorian, as it has the potential to fill in some of the new sequel trilogy's back story and even last night's episode (1.07) has a curious parallel between one character in it and a character in The Rise of Skywalker!}