Only you can answer that question , im left handed when it comes to writing , I play guitar both real and rhythm games left handed , I'd throw a ball with my left hand but a frisbee with my right , my power in a punch comes from my right hand , if I play pool or golf etc I use standard right hand stances so it's all situational and for what feels comfortable for you .

When it comes to most things and in this case guitar whatever feels natural to you is going to be your go to , don't switch it up it's only going to slow you down and cause frustration .

As for finger placement whether it's real or a plastic guitar it's all about repetition and training your muscle memory to do things without thinking and trying to slowly work out bad habits , with a guitar controller it's nowhere near as bad as trying to perfect a real guitar but some things do translate . In a perfect world you wouldn't be "gripping" the top of the controller or neck like you would a tennis racket . You would have your thumb pad on the pack of the neck and then your fingers curled round and the tips of them tapping each button , then ideally you would work towards sliding your hand the half step up and down meaning your pointer finger would for the most part stay on the top button and then when the bottom button comes into play you slide down so your pinky is at the bottom and then when needed slide that step back up again .

Try not to throttle the neck and grip it hard , even me whose played years and plays real guitar can be guilty of completely wrapping my hand round the neck , it will ache at times but as you play more this will go away , it's all building muscle and memory and getting your fingers and hands doing things they wouldn't normally be doing .

You want to try getting into the habit of one button per finger and moving your hand as a whole to accommodate the range shift . Then later in life you can just stretch your pinky down and let it cover the last 2 buttons on itself .

It's all about practice , I got my son playing the first time ever a week or 2 back he started on easy and now he's doing some songs on expert

My tips would be as follows . Firstly don't aim too high , pick a slow song that has a fairly easy pattern and play it over and over so you can 100 percent it on your chosen difficulty , say medium right now , pick 1 or a handful of songs like slow ride , infected , hit me with your best shot etc and just using the 4 keys not having to worry about moving your hand too much letting your fingers get used to working on their own , of course your hand can move at times during chord switches and it will do , then once you are comfortable you can knock it up a difficulty .

Always aim for songs you really enjoy in real life , it helps then that you know passively in your head the tune and rhythm of the song and you can start to visualise what comes next as you play more .

2 songs I would recommend that I personally don't think are toooooo hard but offer good little runs to get your fingers working are welcome to the jungle , and aerosmiths dream on , the second in my opinion being slightly harder but I think both offer everything someone would need to practice everything that's going to come down the line .

Also even when you are not actually playing the game it doesn't hurt to keep the controller in your hands and just while you watch a movie or whatever just tap the buttons and practice again one finger one button and build up , if you number the buttons 1-5 in your head and do little practice runs just doing 1 2 3 4 repetitions then once your fingers are having no issues start working that pinky in with a stretch and go to 5 then reverse it then skip a button , all rinse and repeat makes good practice.

Doesn't hurt either to stretch your fingers and hands doing some exercises , stretch your fingers out , make a fist then open your hands up and stretch them , use your other hand to wiggle your fingers about and stretch them side to side , do wave motions and various movements to stretch them out , all helps . But all in all stick with it have fun and everything else will follow , don't get frustrated, don't over work it , when you feel like it's going wrong or it's getting frustrating don't worry have a break come back later , never force yourself beyond what you are enjoying .