I'm garbage at ARMS, how do I get better?

[Shiken]

Learn the perks to every character. Also if you hold the dodge button, block button, or land after jumping without attacking, you will activate the perk to your arms if you attack right after.

Personally, I like to jump and throw single punches with my arms activated. I only grab when my oppenent is off guard or blocks way to much. Also try to time your overload during one of your opponent's attacks, or they can dodge or block it.