Baldur's Gate 3

While everyone else is giving a totally correct answer of "do what you want and what seems fun", for sake of completion I'll give an alternative. If you're looking for an optimal choice with respect to character power, there's one specific point of the game where multiclassing is not recommended: exactly at level 5. At level 5, each class gets a substantial power boost. Martial classes get extra attack and casters get access to third level spells. Generally, these jumps in power are greater than whatever you're getting from adding an additional class. Likewise, multiclassing at level 4 delays a feat/ability score increase, but this is a bit more manageable.

That said, this is pretty min-max-y. Yeah, being at Cleric 3 / Druid 2 would be weaker than level 5 in either class alone, it won't be non-functional. You'll still be able to play fine even if your build is slightly suboptimal. Even on tactician, as long as everyone in your party isn't crazy multiclasses, you'll be fine as the party can cover for any shortcomings on your behalf.

Lastly, you can always respect. At level 3 and want to dabble but still want to be built optimally at 5 or 4? Go for it, and then visit Withers when you level up for a quick respec. Bottom line, play how you want, experiment! You might temporarily miss out on a power jump, but it's not crippling and you can always readjust if you feel underpowered

Each time you level up you can choose which class to assign that level to, so on your example, if you take druid you can go back to level up your cleric any time.

As for when starting a new class, it depends on what you want to accomplish, there's a comprehensive guide on the wiki.

I multi classed at lvl 2 taking 2 lbs of druid before continuing deeper in ancients paladin. (I know off meta pick but I find it compelling) you can always respec for 100 gold so just do it when ever you want and tweak the difficulty until you have fun. If you can't get to that point it's time to respec.

Everyone has done a good job covering the pros/cons of what you sacrifice at each level. I'd just like to mention that the specific classes can matter too. For example, I like multiclassing monk on everything because punching is fun. In a situation like that, getting tavern brawler at level 4 is my power spike, so I stick with the main class until 4 then throw some levels in monk. It's possibly to find pretty goofy combinations between classes and only 100 gold to respec so there really isn't harm in messing around and testing builds out.

When should I multiclass

When you are chasing a specific feat, ability, or trait that comes from multi-classing. You should know why you're doing it, and in knowing why, you'll know when.

What are you looking to get from druid? Will need more details if you want a response on what would be optimal. Also, you can multiclass at any time, but I'd recommend against it unless you know what you're doing. Then again, this game is probably easy enough that it won't cripple your playthrough or anything, and you can respec for 100 gold should you mess something up.

And yes, you can choose which class to put a level into each time you level up.