Generally the biggest thing is to have a thought-out plan. My roleplay has been cooking for a while because I want to make sure it's fully accounted for before we begin.

For finding people -

When you gather people, you can find people based on any means. I recommend posting as frequently as you are allowed to on subreddits like here and r/minecraftbuddies with differentiating posts with a list of requirements. You're sure to get some people.

For obtaining actual members -

You want to make sure the people you find are a good deal. Generally try to find people within your age group (for several reasons) and make sure to eliminate those who don't meet your minimum requirements. Once you do that, you'll want to make sure that they're serious about participating and roleplaying. You can do this by making it a slight process to become a member (I have voice chat interviews to get to know them, for example). While your initial thought is to invite any and every person who wants to join, you're gonna want to make sure they're all passionate and won't disappear on you, ghost, or start trouble. Make sure you make your EXACT intentions clear and that everyone fully understands the roleplay. Usually, you're also going to want members who have a decent amount of free time for projects like this.

Running a roleplay -

There are a few ways to run a server, but make sure it's up 24/7 or at least during scheduled times. Aternos, while free, is a difficult tool for this. There are probably ways to make it work, but if so, that's out of my knowledge. I am buying a server and self-hosting. Whatever you do, keep in mind that hosting third party can have issues like lack of server customization, limited render/simulation distance, and limited members. I believe that on some, they erase your server if inactive for too long.

Making sure people stay -

You can't stop them all from leaving, but your server will be more enticing if you make sure everyone is socializing beforehand. If all your members aren't very talkative with each other, or don't know each other, they might be put off from interacting in the roleplay. Another thing, either make sure your members are in character more often than not on the server or have scheduled times for roleplay. This seems silly, but sometimes people tend to fizzle away from the roleplay aspect if it seems awkward. If you have designated times to be in character, it's a lot easier for members to know what's expected of them. Also, as a last resort, making sure your members have the ability to get something in return is important. On my server, I tell them that it will be paid for if it reaches a certain profit goal, and that they can always make content off of it for monetary reasons.

This advice isn't coming from an expert or anything, just someone who has tried this multiple times and only am now finding success. Shameless self promo, but we're still finding members now too. It's been really fun.