Hello! Some friends and I, are going to be playing DnD for the first time soon (We are going to be playing online)

I have played a one-shot once, about a year ago, and two other players have as well. But generally we do not have any experience. I was looking at buying the Dungeons & Dragons 5th Ed. Dungeon master's guide, is it worth it? Will I need it?

So far I have been looking at some free one-shot campaigns online, but I haven't found one that accommodates 6 players (7 people total, including myself the DM) Any suggestions for a couple good ones?

Lastly, we have been thinking of using Roll20 for the platform we are going to play on, is it good?

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That's about it, any help is greatly appreciated!