This is not financial advice
From my personal experience, the motivation of doing the research, tends to be easier if you have already invested a small amount (this is only my personal opinion, you can do the research without investing a small amount). You can start with whatever amount you want. Remember, only invest what you can afford to lose. The chance of you losing everything is very small, as long as you invest in something where you have done your research or in one of the top 10 coins. The safest place to buy your very first amount of bitcoin, is through Coinbase or Ethos wallet. Even with your first investment, make sure you buy in at the right time (buy low). You can read more about this in step 6 or in the members section "How To Invest Successfully" (require membership - read how to get a free membership here).
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Why start with bitcoin?
Bitcoin (₿) is the number one cryptocurrency and a worldwide payment system. It is the first decentralized digital currency, as the system works without a central bank or single administrator. Bitcoin is arguably the safest investment to start with.