You can see it ranked in the top 50 on CoinMarketCap, make sure the Altcoin you choose has a large trading volume. What needs to be underlined is the difficulty of determining the investment period, if in the medium term like your target does not achieve profit then you have to be patient for the long term. You have to be very observant in making decisions.
not just looking at the rankings. but research is also important to do. Doing research with technical analysis will provide an overview of the short, medium and long term, so that we can determine which coins are good for short, medium and long term investments.
The large trading volume and being in the top 10 of CMC is also a good choice. But make sure to buy at deep prices.
As you suggest, for those who are new to trading, its best advice to choose the top most coins which are high in trading volume on best exchanges as this will be a safest and the correct move in the beginning. Invest little in the beginning. Choose Bitcoin as the very first one. If you wish to choose coins that are less or equal to a dollar, then choose Polygen (Matic) which is the best choice for now to trade on mid term.