Resources for app design and UI/UX?

Just pick one you like then dont look at anything else. Too many distractions in the market right now. I've been on Sketch since March 2017. And before that Photoshop for 10 years.

It really depends on who your target audience is for your concepts. Do you need to just get feedback on a solution based on a rough concept? A/B test a non-interactive design? Do you need to hand off a high fidelity design to a remote contractor?

As a UX designer and developer I have my preferences, but for non-tech/non-devs I recommend starting with whatever is easiest. Hand drawings, Powerpoint, or whatever you've used for other work that can do light duty wireframes or image cutting and pasting. Don't get hung up on tools, focus on what you need to get feedback.

That being said, for rough concepts a basic wireframing tool like Balsamiq is easy to pick up, and cheap. There's many online tools that are similar. Chances are a wireframe will get you most of the early feedback you need.

If you need to do polished mockups, like when working with off-shore contractors, a graphics tool works well. Sketch is great, I've used it about 3 or 4 years, but have switched to Affinity Designer recently and love it. Either work well, and if you need to do actual prototypes you can combine them with something like Invision or Proto.io to add interaction. Adobe is offering it's XD (Interaction Designer?) for free right now, I've found it excels at doing quick mockups with basic interactions. It seemed easy to learn, but I've used Adobe design tools since the 90s...

Been using Sketch for over a year and I love it. Use it for everything from UI mock ups to creating my app store screenshots. Have used https://www.sketchappsources.com for templates when I'm feeling lazy

Try: https://www.uxpin.com/