If you're a beginner, it's better to focus in buying more bitcoin and less than altcoins
Beginners can focus on buying altcoins as well so long as the altcoin they choose to invest into have long history and did not experience a pump recently. Focusing only on trading bitcoin would not help them gain more knowledge on trading cryptos.
Aside from actually trying to learn and study first before going in, I always suggest that new traders should always use only a very small percentage of their total trading stack(probably 5%, depending on how much the total stack is) to start their trading.
That would depend on how much money they got on their portfolio IMO. If a beginner's portfolio is less than 100$, it is better to start with 15% of their money in crypto rather than 5%.