Lost Judgment

You should unlock this naturally as you play. Spend your skill points using the ‘skills’ app on your phone. You can also see your total number of skills in the top-right here.

There are a few things you can do to speed this up:

• prioritise the cheap skills
• invest in ‘battle bonus’ skills (under ‘stats’), and get into as many fights as possible
• eat at restaurants regularly and choose dishes with a skills point boost

Gear that gives an SP boost:

• Secret contacts (5% boost) - ¥15,000 from Kinka Pharmacy
• Training Band (10% boost) - ¥78,000 from Ebisu Pawn Kamurocho
• Training Long Pants (20% boost) - prize for completing Gauntlet 1
• Training Innerwear (20% boost) - Squirrel Search (Near the W. Shichifuku Poppo in Kamurocho).

I got this early on in chapter 2 by exploring a lot, and following the School Stories questlines.

Note that the boxing skills (from the DLC) do not count towards this.

Over on gamefaqs, CyricZ's excellent Lost Judgment guide includes a list of all skills, and how to unlock them:
https://gamefaqs.gamespot.com/ps4/320117-lost-judgment/faqs/...