Short answer
Outside of a Free Fly you need a Game Package to be able to play Star Citizen permanently. A Standalone Ship is just a single ship and is not enough to access the game if you don't have a matching package yet.
What is a Game Package?
If you have decided not only to test Star Citizen during a Free Fly, but to play permanently, you need a Game Package. Important is the distinction: A Game Package contains the game access. A Standalone Ship is only a single ship and is not enough if you do not have a game access yet.
After purchasing a Game Package you do not have to pay a mandatory monthly fee to play Star Citizen. There are optional issues such as ship upgrades, additional ships, skins, subscriber offers or cosmetic items. These things are not necessary to enter the game with a Game Package. Just as a beginner, you should understand your first package as an access to the game, not as a final decision for your entire future in the 'Verse.
Which Starter Pack is enough for beginners?
The right Game Package depends less on which ship is objectively "the best one", but on how safe you are when buying and what kind of game you are most interested in. It is important to get started: you need a Game Package to play Star Citizen. A Standalone Ship alone is not enough if it does not have a game access.
If you're still unsure, the most reasonable decision is usually the cheapest available Game Package. This gives you access to the game, you can test the verse and later decide whether you want to invest more money. Star Citizen is a Alpha, and that's why you shouldn't start directly with an expensive package just because a ship looks good in the store.
A small start will give you time to really try out the game. You can fly with other players and test, rent or purchase ships in the game depending on the current state of play. RSI also offers ship upgrades to ships of higher value for suitable pledges, but upgrade paths are not always available and upgrades applied are permanent.
The currently available Game Packages can be found in the official RSI store.
You can't decide?
If you're new to Star Citizen and want to try out multiple game activities, Avenger Titan is a very solid all-rounder recommendation. It's not the cheapest starter ship, but it offers many advantages for getting started: you have a small cargo bay, can try simple deliveries and cargo tasks, are not completely defenseless in battle and have a ship that is suitable for various first activities.
This versatility makes Avenger Titan attractive to many new players. You can fly with it's delivery missions, gain first cargo experience, try out simple bounty missions and find your way around the ‘Verse' in general. In addition, it has an interior with a bed, which makes it a bit more comfortable and immersive than very minimalist starter ships.
The downside is the price. Compared to the cheapest Game Packages, Avenger Titan is more expensive. If that's too much for you or you're not yet sure whether Star Citizen really suits you, take a cheaper Game Package or wait for a Free Fly event. You can still upgrade later and only pay the difference between your current ship and the target ship when upgrading.
Important: Availability, package names and content can change in the RSI store. If a particular package is not available right now, check the current Game Packages overview and not just buy because of a single image or offer.
So the most important recommendation remains: don't buy more than you really need at the beginning. A more expensive ship doesn't automatically make Star Citizen better if you don't like the game feeling, performance or the Alpha condition at the end. Start carefully, test and decide later whether you want to spend more.