It depends, mostly on what you want to do eventually. Are you new to Magic as a whole, or just new to Arena?

Nothing is going to rotate out of the Standard, Brawl, or Alchemy formats when Outlaws of Thunder Junction is added to them. Cards you acquire for those formats now will remain legal in them until August 2nd, when Bloomburrow is added. At that time, some older sets will rotate out. If you plan to get involved in one of those formats, look at those pages I just linked for them to see what sets will be rotating out, and try to build decks with cards from the newer sets.

Larger formats like Explorer, Historic, and Timeless never rotate. Cards you acquire for them will remain legal indefinitely (although there’s always a risk that some will get banned).

If you’re planning to mostly play Draft (or Sealed Deck) then it doesn’t really matter what cards you acquire (the way the Arena economy works, you can’t sell or trade them). But in that case I might recommend waiting for Outlaws, rather than trying to learn Murders at Karlov Manor now, since it’ll stop being draftable in less than two weeks.

If you’re totally new to Magic, Jump In is probably the gentlest introduction Arena has to offer, and there’s no need to wait to do it. It’s structured so that you’ll see cards from several different sets, and there’s no metagame to learn. You should also definitely check out the Midweek Magic events that run from Tuesday to Thursday every week. They won’t build your collection very quickly, but they are (usually) free and (usually) don’t require you to bring your own cards. They’re a great low-stakes environment for learning how Arena works.

On the other hand, if you already feel at least somewhat comfortable with drafting, that’s probably a more direct route to building a collection that will let you compete in Standard/Alchemy/Brawl. (Working up to being competitive in non-rotating formats will take longer; you should focus on one of the rotating formats to start.)