To use a tool go to your rucksack and drag the toll you want to use into the top box. Then simply press y to use it :)
email the harvest moon ds expert at hmdsc.expert@gmail.com
-- I promise it...
There are no codes. In Harvest Moon DS there were GLITCHES, but they were "fixed" in Harvest Moon DS Cute. But you don't really need to use codes anyways, but if you need any help, use this...
If you mean tea leaf seeds then you buy them from Marimba Farm in the summer. But if you mean just tea leaves then I don't know where you get them. Sorry!