“First and most importantly, you need to make sure that all your students are accounted for.”

You're an administrator at a school. You're giving a presentation to the teachers about your school's plan for what to do in an emergency. You start by telling them the most important point:

First and most importantly, you need to make sure that all your students are accounted for.

First and most importantly, (sentence)

People use words like "first", "second", "third", "last", and "finally" in presentations to talk about items in a list, or steps in a process:

First, it's extremely easy to use. Second, it's 30% less expensive than the leading competitor. And finally, our customer service is the best in the industry. 

But the first item in a list is often special. So we combine "first" with some other word to introduce the first item. In the example above, the speaker uses the phrase "first and most importantly". Another phrase with a very similar meaning is "first and foremost":

First and foremost, it's extremely easy to use. 

make sure that (something happens)

To "make sure" of something means to check on it, so that you know that there are no problems. For example, if you're worried about eating meat that's raw inside, you can say:

Make sure they're cooked all the way through.

"Make sure" can be followed by "that ___":

Make sure that all the lights are turned off.

Make sure that we didn't forget to include any of the forms.

I made sure that we weren't being watched by anyone.

(something) is accounted for

When something "is accounted for", it means that you know where it is. You usually use this phrase to talk about a group of things, like all of the students in your class:

Everyone's accounted for.

This means that you know where everyone is. They might not all be with you, but you know that, for example, 28 of them are in your class, 1 is at home sick, and 1 is in the school counselor's office.

Either people or things can be accounted for:

Before you leave, please make sure that all your valuables are accounted for.

The opposite of "accounted for" is "unaccounted for":

Two soldier have been killed, and three more are unaccounted for.

The phrase "make sure ___ is accounted for" is common, so you might want to memorize both parts of it together.