Get the game for your mobile device. It is available in languages other than Japanese but not English right now. So you go to your respective App store and download it. I am not going to cover the Line installation because I do not have a Japanese Line account.
On the KOF98UMOL main Japanese page there are links to iOS and Google.
After installation and launch, the game may update before you start to play. In that case, there is a Shermie mini game.
The game has a somewhat long tutorial. Characters will pop up and guide you and tell you what to press to advance the story and game. The game is a lot of targeting, pressing and timing.
Just pushing the buttons targeted will get you until about Level 10 before you can start to freely play the game.
As you progress through the game and leveling up various things will unlock. For example, but not in order: leveling up character, boosting stats of character and equipment, shops, bonus type states and dailies. The game has all kinds of achievements and rewards. This actually makes the game easy to play without paying. There are rewards for leveling up, logging in, powering up, beating a level, etc.
When you play the stages and beat up an enemy you get all kinds of misc drops. Cash, scrolls, materials, treasures, cards, medals, car parts. These are all crucial for leveling up your guys and getting cash. Generally speaking, the scrolls are the only thing worth selling. Most odd parts are automatically sold when you enter the store.
Right now I am playing through the game for a 2nd time because my previous phone died. I plan on breaking down the individual sections of the game because there are so many of them.