If you have a mixed group of experienced and green players, you can save yourself a lot of trouble by engaging the veterans to help the newbies. Let them suggest stuff and guide their apprentices, and even throw in some game-tangible bonuses like mentoring XP. And if a mentor happens to screw up and put his apprentice in a tight spot, he will probably do a lot to help them out as well, and you as the GM will have more sympathy from all players if you have to intercede and fudge stuff to keep the newbie in the game.
"Get out of trouble" tokens
Another suggestion, which can be used together with the one above, or as a standalone mechanism is; you can use some kind of "Get out of trouble" tokens, and give a couple of them at the beginning of the game, only to the green players. State that these tokens will allow the player to undo their disastrous actions, but they will never ever get refreshed, so they better learn from the experience. The veteran players will be more understanding if you make this a part of your rules, rather than fudging stuff from the top of your head.
"Better" may mean different things to different groups, so it depends somewhat on the play style of the GM and other players.
If you have a group that is heavy on the role-playing side, don't be afraid to step outside your comfort zone. It is sometimes a challenge for new players to speak "in character" because it feels a little embarrassing when you are new to the game or group, but getting comfortable with that aspect of role-playing can lead to some of the best in-game moments. Don't worry so much about trying to do an accent, or speak with some other affectation; just use your own voice but try to think about how your character would react in the given situation. Instead of speaking in 3rd person like "My character says...," just speak in the first person, "I am honored by your presence, Duke NPC". Check out role-playing podcasts that discuss 1st person vs 3rd person. Google "Fear the Boot" and peruse their role-playing show topics.
If you have a a group that really enjoys tactical combat, it's important to understand how your character's feats/spells/maneuvers can aid the other characters on the grid. You might want to do some Google search on class optimizations / builds. Read through the combat section carefully. Understand how flanking, feints, aid another and other in-combat maneuvers can give you or you allies bonuses. If you are a spell-caster, research the use of buff and de-buff spells beyond just damage dealing spells. As an example, "Grease" is one of the most powerful utility spells in the game.
In terms of helping the GM, listen to the descriptions given as the GM narrates a scene. Ask questions about the details of the objects and surroundings of the scene. Ask about your other senses (smells, sounds) as the GM may sometimes to forget these details and the interactive give and take of noting these small details can really help a GM's creative juices as well. Honestly, as a GM, I am brought up to another level when the players ask leading questions. Sometimes I don't have all the answers, but other times it suddenly occurs to me that I can add a clue that I might not have otherwise thought to include or gives me inspiration on how to to build more atmosphere.
Don't be afraid to ask questions of the more experience players, but remember that it is your character, so have him or her act as you wish. You don't always have to act in accordance to their advice or desire.
Lastly, thank the GM for their game. It takes a lot of prep work and it is often thankless. At the end of the night, note something in the session that you thought was really cool, or particularly fun about the session. If you are really comfortable and friendly with the GM, you can sometimes offer positively framed constructive critique, but this is a very tricky path to tread and you really need to have a strong trust relationship with that person.
Breakdown the priorities of each option, and let them pick their poison.
Ignoring the individual mechanical benefits of certain multiclassing combinations (Hexblade + Paladin, Rogue + Barbarian), almost every class and subclass can be analyzed for specific trends. By pointing out why someone would pick one option over another, for something they find interesting, you can almost guarantee they won't be disappointed by whatever option they choose.
Right now, your players are likely suffering from Decision Paralysis, and the solution is quite simple: Make Things Simple.
Look at each class and come up with two for each lists for each:
For the items in those lists, organize them based on priority or importance.
Try to accommodate any changes that you might gain from subclasses and features. For example, the Rogue doesn't inherently gain magic abilities, but it can gain the option of manipulating magic items from Thief, or casting magic spells from Arcane Trickster. Try to summarize the class as a whole.
So, for example:
What this allows someone to do is determine whether or not a Rogue is a valid option for themselves.
Many Wizards might not like the idea of being dependent on weapon attacks, despite liking the idea of a "roguish" caster, and may look to other option to get the solution they're looking for.
On the other hand, a Wizard who's looking to improve their stealthiness and is relying more on attacking (say, a Bladesinger) can easily identify that the Rogue would be a solid option (compared to something like a Monk or Fighter).