I'm a beginner in learning Japanese and would like to know what's better to start with first, hiragana or katakana, and why?
It almost doesn't matter, because you're going to be learning them both quickly enough that it won't make much of a difference which one you start with. And you'll be using them both extensively every day, just as you use both lower case and upper case every day in English.
But still, it's a sensible question. Which one should you choose?
You can make arguments either way. I started with hiragana, and that's what makes the most sense to me. In a way, hiragana is more basic; it's more common, and most of the Japanese vocabulary is more normally written in hiragana than katakana. But starting with katakana could make sense, too! If you're in Japan and barely know any Japanese, knowing katakana will let you recognize at least some words borrowed from English. And unless you're Chinese, your name will most likely be written with katakana.
But really, you can do it either way. Just learn to write them both as soon as you can, and make sure you learn to write them the right way, with the proper stroke order.