I have 3 ST and I've seen some cool items you can get for them. But the cooler ones are like 6 ST+, how would I earn more?

win tourneys maybe or forum events

DoGM hotseats, sponsored/gm events, etc. etc.

Try googling your question before you make a thread.

Earn Shiai tokens through most GM or GM sponsored events.

Any GM hosted event will most likely give out a shiai prize. 2 to 3 ingame events are hosted daily, just watch out for them.

You can also find GM events on the forum that may or may not give out Shiai tokens. These will normally be held around holidays, however some are hosted randomly throughout the year.

Rarely, you can find a user/clan/org hosted event that the item forgers or GM team have sponsored. These events will be hosted mainly by the person/clan/org hosting it, however the GMs will play a part in the prizes, and will also add an official feel to the event.

GM events.

GM events are a common way of earning Shiai Tokens.

Win GM-Tourneys in-game, win Forum Events and that's it.

I Got 2 ST Right now, where is the shop to use it?

Here

This is the thread were you can redeem your items for shiais.