(Spreadsheet down below)
Every Accessory grants you Magic Power, depending on its rarity, but only while it is in your accessory bag. So having accessories in your inventory or anywhere else is completely useless. You can put your Magic Power to use as soon as you talk to Maxwell for the first time (next to the Alchemist in the hub). He'll give you the option to choose from several Powers. Those Powers give you a certain set of stats, and they scale with your Magic Power.
At the start you will only have very few options to choose from, but you will unlock more options through simply increasing your combat level or by learning Powers from Power Stones.
The best free powers to choose will be Warrior at first, and Commando once it's available to you.
The way you learn a new power through Power Stones is by clicking the "Learn Powers" button in Maxwells GUI and putting in 9 of one of the power stones. I heavily recommend you get Bloody (Beating Heart) for melee damage and Sighted (Ender Monocle) for intelligence. They are very cheap (100k/700k total) but very useful.
Other good (but more expensive) options are Itchy (Furball), Hurtful (Magma Urchin) and Scorched (Scorched Books) for Attack Speed/DPS, Silky (Luxurios Spool) and Shaded (Dark Orb) for single hit damage and Bizarre (Eccentric Painting) for Mages.
Finally there's also Tuning Points. Every 10 Magic Power you get 1 Tuning point, that you can use to boost any (combat) stat you like. It's recommended to put your tuning points into hard-to-get stats, one of the most useful being Attack Speed.
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