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You can start by buying bitcoin on an exchange that provides your currency and start buying it, but I think the average person is successful by buying on coinbase. After that, you can just trade by exchanging the bitcoin that you have on several altcoins, which you think has great potential for the future.
Do you mean that if we buy bitcoin in the other exchanges we will not be successful? It is ridiculous mate and I don't think so.
Even though we buy coins in a known exchanges or using them in trading but we never have 100% guarantee that we will become successful cause it is depending on how we manage our trade. Coinbase is not only an exchanges around, we can use Bittrex or Bitmex in trading, they are known exchanges also in the market.