Every beginner should start with Bitcoin. The cryptocurrency industry began with Bitcoin, the original coin. The industry owed everything to Satoshi Nakamoto and to his creation, Bitcoin.
Before you move to other coins or tokens, you have to first study Bitcoin and what it has attained so far. You will have missed the most important lesson on decentralisation and privacy if you did not study the reason Bitcoin was created.
As much as I love Bitcoin, I think its future upside is more limited than many people expect.
There's no doubt Bitcoin will remain the biggest cryptocurrency, but will it ever reach $1 million? Maybe, but I'm sceptical. Bitcoin has already achieved mainstream adoption, and while I think it can continue to grow steadily, the kind of explosive gains we saw in the past are much less likely. I hope I'm wrong because I own some Bitcoin myself, but I just don't see that kind of parabolic growth happening again.
"What altcoin should I hold?" is a difficult question, and it's not one I'd ever answer with a specific recommendation. Altcoins are extremely volatile and can be vulnerable to bugs, exploits, or project failures. If you decide to invest in any altcoin, make sure you do your own research first. Even then, stay alert to market conditions and be prepared to sell if the market is no longer supporting the price.