@ExponentialDecay Yes to all especially the RA point. A lot of schools don't allow undergraduates to be a TA, but if yours does fully take advantage of it.
Also start putting together a CV and build that CV up with research, volunteering, teaching experience, etc. Also begin to put points together to write a good statement of purpose, granted you won't need it until you start doing applications, but it's nice figuring out what your philosophy is so you don't have to do so last minute
@HTM18 I second that 100%, in my experience GRE scores can make or break an application.
Also, start identifying professors or others of which you think would accurately know your skills for when you need letters of recommendation. Let your advisor or professors you trust know that this is a path you want to pursue, as they may be able to point you in the direction of some good opportunities.