To gain supplies, you must build a production building (the second tab on the build menu). A production building cost mainly coins and population which can be gained from residential buildings (first tab on the build menu; the most common in the start of the game is a hut). Once your building is built, you can collect supplies from it after you set it to an interval and come back and click on it to collect it (then you set it again and repeat process
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Hope that helps!