Welcome!
Both are good questions with answers that vary a lot depending on your goals, time, and a whole range of other factors.
On the intervals, most people do best doing their reviews around when they are first due, but this is much more sensitive for the shorter intervals (eg if I’m 2 hours late for a 4 hour review I’m much more likely to get it wrong, but if I’m 8 hours late for a week review it doesn’t much matter).
Most people also do best on wanikani with more than one session of reviews per day, since it lets you go for accuracy, and progress faster. If you want to optimise intervals beyond that general advice, here’s a long and amazing guide: My Journey of 368 days (+ The Ultimate Guide for WK 📖 ).
Wanikani only teaches you kanji so you need other materials to learn Japanese. And when to bring them in is a matter of judgement for you! You can start right now if you have time and energy, or you can wait a few levels till you have a little bit of kanji knowledge to help you feel comfortable. Views vary on this but there’s a lot more people on the forum saying they wish they’d started earlier, compared to the other way round, so if you’re keen now then now is a great time to start.
Best of luck, hope it’s a fun journey!