I have started two leagues and here are the settings I have used
Platform - Sleeper, allows for the most control and customization, keeps track of picks for 2 years in advance
Teams - dependent on number of owners, but 10-12 is typical
Roster - I prefer 1QB,2RB,2WR,1TE,2FLEX,1SUPERFLEX,15BEN,3IR
I’m not a fan of taxi squads, which are another option to add to roster that are used only for rookie or second year players, I’d rather just have more bench spots. Bench should generally be 1.5-2 times larger than starting lineup. Definitely prefer superflex, makes value of qbs higher and more players are used. Also, no kickers or defense, hard to plan for long term and aren’t really good in terms of trading.
Scoring - this is where there can be a lot of differences in dynasty. I prefer 4pt passing touchdowns and 0.5 ppr for all positions, but you can change the points per reception for each position if you want. For example, can have wrs and rbs 0.5, but TEs get 1 ppr. Otherwise sleeper does typical scoring for yardage by default, decimal scoring with 0.1 pts for receiving and rushing, 0.04 for passing(1pt every 25).
Some other advice, if this league involves money, collect money for the current year and next year, makes owners more committed to the league for the long term. I would have a trade deadline around week 10 or 11 so teams can decide what direction they want to go for the rest of the season. A huge thing for me, don’t have league vetoes, allow all trades to go through right away unless there is collusion. And I would use FAAB for free agency, I prefer to have $200 budget where teams can use $100 in offseason, then I set everyone’s FAAB to $100 right before season(so $100 offseason, $100 in season budget).
In terms of the draft, you have a couple of options. The easiest would be to wait until after the NFL draft and do a draft with both veteran players and rookies, or you could do separate rookie and veteran drafts. I could give more details on how to do separate drafts if you want, but the combined draft is easiest, especially if you are new to dynasty. Once you complete the startup draft, I like to have 4 rounds for the rookie drafts you will complete in future seasons.
Hopefully this helps, can respond with more questions or if you need clarification.