There are a lot of tips for beginners in Android development. Not all of them will be applicable for you. Honestly it takes practice and trying out a lot of different things to see what works for you and what does not. Here is a good article with a list of some great tips and tricks.
As a developer/designer, there are some of the best practices and guidelines that you can follow to make your app look good, or make it faster. Again, it depends upon you what to follow and what not.
Material design is a set of design guidelines created by Google which include everything from key lines, metrics, fonts, colors such that the user experience becomes attractive and intuitive. There’s a good YouTube Playlist for Android Development patterns where devs talk about common design and development patterns.
Let’s say that you are trying to implement something for your app. Chances are, that a set of functionalities that you are thinking of developing yourself has already been developed by someone else and is kind enough to share their code for free. This is the true power of open source. Here is a good list of open source libraries for android developers.
There are a lot of good and experienced android developers on online communities. They can help you out if you are stuck or need help with anything. Reddit is always a great place to start.
