i got told its usually the first chord of the song or something but im still new lol
Well, a general rule and to make it simple, is that is almost always the chord that appears most frequently in the song.

Cause a song can start with the C#m chord, but still be in E, if you get my point.
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90% of the times you should look at the last bar of a song, or the last note even. Most songs end with the basenote of the scale or the 'I' chord because it makes a song feel 'finished'.
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Generally, it's the first chord used.

But you can check by working out which notes are used in the solo/song, and finding whch scale they're in.
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