If your account is only a month old you really don’t need to worry about prestige. That’s mainly for competitive level alliance quests (map 5 and 6). But if you really want to know, it’s the average PI (number each champ has) of your top 5 champs without masteries. You can find prestige calculators online or in the Line app/ClanHQ as a couple of examples.
Just more thing. Most of the calculators are designed for 4* or 5* with high levels. Very likely you wont find one that works for you, assuming that most of your champs are 3*.
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I know especially because it has such an impact on the game