When should I multiclass? For instance: I have a very low level level cleric that I’d like to try a level or two of Druid with. Is there a specific level where I should take a level of Druid, or does it not matter? Do I do it way later, or earlier?
If I take a level of Druid, can I go back to leveling cleric? Or does cleric stop at that level?
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
I want to add in, though, that these breakpoints also work for multiclassing, as points to shoot for in a class before dipping out. The other thing to add is Subclasses. The subclasses in 5E are pretty powerful, and most classes get their subclass at level three, so that's another breakpoint.
So what you have to bear in mind are these things:
Feats/Ability Score Increases come every 4 levels, and you don't want to miss out entirely. You can get away with one, but shooting for two is recommended, which means ending up with 4+ levels in two different classes, or 8+ levels in one class, or 6+ levels in Fighter (they get a Feat at level 6).
The big breakpoints mentioned, as in level 3 and level 5 primarily. This is especially important for Martial Classes with their extra attack. If you want to be hitting things with a weapon, you need to get Extra Attack from somewhere.
Consolidating Attribute dependence. Mixing and matching martial classes is fairly simple because they all rely on either STR or DEX. If you're mixing caster classes, make sure you know what you're doing with their casting stats. Druid and Cleric, for example, both use WIS, but if you mix in WIZ-levels you'll probably have to stick to spells that just work, like Shield, since your casting stat for them (INT) won't be high enough to land them reliably.
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.
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.
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.
I like the idea of Druids, plus- the circle of land is a good sub class I think. Sort of a rp-ish idea. Also- does it matter which class starts? Would it be better to start as Druid then get cleric?
In this case, doesn't matter which you start with. For some classes, it matters, for example fighter only gives heavy armor proficiency if you start as it, not if you multiclass into it, but there's nothing like that here. It's mostly a matter of how much you're fine with delaying those higher level cleric spells. I'd suggest picking up Shillelagh with your druid levels, getting both your weapons and spells scaling with Wisdom.
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.