There's a guy who was so nice
If he's showing you his dick and pressuring you for nudes without any indication from you that this behavior is wanted, then it honestly does not sound like he is being nice at all. He's being polite, and telling you what he thinks you want to hear, so that you will engage sexually with him. AKA, he has an agenda - which is manipulative, not nice.
I'd additionally be wary of providing information over any app targeted toward teens and children. Many pedophiles pose as adults on there, looking for people to groom. Even if they send you a picture, they could be 20+ and you might not be able to tell from just a junk shot. I could send you this photograph:
and tell you I am 17, and you might actually fall for that. I routinely get mistaken for a teenager, asked if I go to the high school, and carded at bars. I am 32.
You also need to be mindful that if you are underage, it could be straight up against the law in your jurisdiction for you to send nudes to others - even if they
are your age. Some teenagers are being charged with shit like disseminating CSEM ("child pornography") just for sending their
own pictures. At the end of the day, if you're a minor, you are still very much a child and you shouldn't be involved with people who want you to send them sexual pictures.
It doesn't matter if they're your age or not, they're engaging in activities that are illegal at worst and unhealthy at best.
I don't even know how to say no to this.
If you don't know how to say no, then close the program. If you can't join these friendship apps without being pressured into sexual activities, then it sounds like you shouldn't be joining these apps at all. This is something that is under your control. You can click "X," you can put the phone down. It may take time and practice for you to disengage from your online habits, but it is do-able.
If you feel those urges, this place is a pretty safe spot to come and post instead. We are mostly adults here, but PMs are disabled - people can't message you privately and harass you, and our Community Constitution is very clear about not tolerating abuse.
As far as mental health forums go, this one is a fantastic resource.