im new to ae; need help.

Since you just install AE it is better to watch some course introduction to AE to get understand this application and how its works, my suggestion is to go to https://www.linkedin.com/learning/subscription/products and subscribe for a free trial at the beginning and search for some After Effects Essential Training course.

The first thing you should do is open the Learn section of the Home Screen or look for the Learn Workspace and spend a few minutes going through the basics of the UI.

Then you should check the User Guide and go through a couple of the Beginning After Effects tutorials just to get a feel for how things work. The most inefficient way to start learning After Effects is to jump on YouTube and find a tutorial. Most of the newer tutorials are prepared by enthusiasts and most of them have poorly explained and even just plain lousy workflows. I spend a lot of time on this forum fixing problems for folks that are following an enthusiast's tutorial.

There are also a few things you should understand about After Effects. First, AE is NOT a video editing app. Most of your comps should be one-shot or one animated scene that you cannot create in an NLE like Premiere Pro. More than 90% of my projects are movies or documentaries with hundreds of shots in every sequence. More than 90% of my effects work and animation in After Effects is one shot and under 7 seconds. The only time that there is more than one shot or animated sequence in a composition is when I cannot create the transition between those shots in my NLE.

Preview time depends on your free resources. Most pros only run ram previews to figure out the timing of a shot or animation. These are usually run with the Comp panel set to Auto resolution and the Magnification factor to 50% or less. All we are looking for is timing and framing. When that is good, we add effects, motion blur, and color correction, and then we carefully check a few hero frames to make sure things are OK. Sometimes we'll run a few seconds of Ram Preview at full rez with all effects, but most of the time, you just send the comp to be rendered and move on with the next effects shot or animation. That is basically the same Pencil Test/Ink and Paint workflow that animators and effects artists have followed for more than 100 years. Make sure the blocking, framing, and timing are OK, then fine-tune and render. Even Disney and Pixar, with all their powerful machines, work that way. The fancy software creates shots, the editor puts them together. 

If your system meets the minimum system requirements you can probably get between 5 and 10 seconds of ram preview with a standard HD comp at 50% and Half resolution. To get longer previews, reduce the comp panel resolution or skip frames. 

Converting audio to a video standard 48 kHz 16-bit WAV file is not done in After Effects. You can export from iTunes on a Mac or even on Windows if you have it installed. Many mobile apps have the option to share or export Wav files. If all you can get is an MP3, and it works in AE, then all you have to do is import it.

Let us know if you need more help.