My pleasure.
Of course, it is the "is" part of "sincsenek" that prevents it from being the first word in a sentence. We can, of course, start a sentence with "nincsenek":
"Nincsenek autók ebben a garázsban." - There are no cars in this garage.
And, if we want to add the "also" part, we can either change "nincsenek" to "sincsenek" (and place the new word where it belongs), or we can leave "nincsenek" where it is and add a "sem" where we would place the "is". It is simpler than it sounds:

"Nincsenek autók ebben a garázsban sem." - There are no cars in this garage, either.

And, of course, we can pick what the "also" should refer to: the garage or the cars:

"Nincsenek autók sem ebben a garázsban." - There are no cars, either, in this garage.
"Nincsenek autók ebben a garázsban sem." - There are no cars in this garage, either.

The "sincsenek" versions look like this:

"Autók sincsenek ebben a garázsban."
"Ebben a garázsban autók sincsenek."

"Autók ebben a garázsban sincsenek."
"Ebben a garázsban sincsenek autók."

Because, of course, we can also play with the word order.

And, as a bonus, if we want to mention several items that are not in this garage, or several garages that don't have them, we can do it with the word "sem".

And, tadaaa, we can actually start a sentence with "sem". This is interesting: "sem" can be both in front and after the word it refers to. The two roles are slightly different:

"Sem A, sem B" - neither A nor B
"A sem és B sem" - A and B also not
There is kind of a logical difference between the two. The first one is an OR relationship (a negative OR), while the second one is more of an AND relationship.

Anyway, here is how to use "sem":

"Sem autók, sem lovak nincsenek a garázsban." - There are neither cars nor horses in the garage.

"Autók és lovak sincsenek a garázsban." - There no cars and no horses, either, in the garage.

It would sound weird to put "sincsenek" after both nouns in this last sentence:
"Autók (sincsenek) és lovak sincsenek a garázsban."
No, this is weird.

But if we have a different verb, the situation changes slightly. Remember, "sincsenek" stands for "se(m) nem vannak" (right?), or "sem vannak". So, we can replace "vannak" with a different verb. Let's pick "állnak":

"Sem autók, sem lovak nem állnak a garázsban." - There are neither cars nor horses (standing) in the garage.

"Autók (sem) és lovak sem állnak a garázsban." - There no cars and no horses, either, standing in the garage.

Here, the second sentence works just fine with the added "sem":

"Autók (sem) és lovak sem állnak a garázsban."

This is just fine.

Naturally, we can do the above also when the "sem" refers to the garage:

"Sem a garázsban, sem a kertben nincsenek autók."
Etc.