Planet fitness and showers/rules?!?

These may be dumb questions but I don’t really understand their rules and want to clear the air haha. I’m 16 and taking advantage of their summer organization that is allowing teens to come work out for free. I’ll be 17 in 7 days but am not planning on purchasing a membership until the summer is over and the teen organization is over. With that being said I was wondering if I would be able to use the showers after working out if I am still not a member or have a black card. Also I know they are strict on clothing or enforcing the no shirt rule, but when I work out I prefer wearing tank tops. Are there any rules around wearing tank tops? I have one that reveals a little bit of my sports bra on the sides and also has a short slit in the back that also reveals some of my sports bra, would that be considered too revealing for the rules of the gym? Thanks!