First, welcome to POA. I had many of the same questions starting out and this board is a wealth of information in figuring out the process. The search feature on the board works wonders.
I'm looking to get my PPL but it's a bit confusing what is required in the way of ground school. So first off, do I need ground school?
Yes you need at least a sign off by a CFI stating that you completed some form of ground school training. There are several options...a college class program, a structured ground school where you go sit in class at an FBO, home study courses, one on one with a CFI, and online programs.
are there legitimate online schools? Are they a good idea?
For online there is King, Sheppard, Gleim, Sporty's and a few others. Do a search here on each as they are all a bit different. I did all my ground school online and would venture to say that most here would recommend that route....if not as your primary ground training, at least to supplement whatever other route you decide.
How difficult is the written test?
Not hard at all if you are prepared, which any good ground school program will get you ready for.
Is it hard or just a lot of new things that people don't typically know?
Yes...you don't know what you don't know...hence the ground school. There is a lot of teaching to the test, but there is a tremendous amount of fundamentals that you need to learn during that process.
I'm also wondering, once you get a PPL do you have to renew it or take any further tests?
Not a "test" but you have to have a flight review every 2 years with a CFI to remain current. (AKA: BFR...
biennial flight review). If you lapse...regardless of the time, all that is required is a BFR and you are flying again (assuming a current medical as well).
Once you have your PPL, there are no further tests needed to mainatin your certificate but many continue on for further ratings and endorsements such as Instrument Rating which is a whole nother round of training, ground school and checkride.
Some do the ground school ahead of flight training and get it out of the way, but many will do it concurrently with flight training so you have your CFI to lean on for questions. Ground school is not required to start flight training.
Your best first step is to go take a discovery flight if you have not done so already and talk to a few CFI's and get some advice as what will work best for you.
Good luck!
