If you are applying for your first job, interviewers mostly check that you know the basics and can learn. These are the questions to practise.
Basic questions
What is a cell, a row and a column?
A cell is one box. A row runs across (numbered), and a column runs down (lettered). A cell reference like B3 is its column and row.
What is the difference between a workbook and a worksheet?
A workbook is the whole Excel file. A worksheet is one tab inside it.
What is a formula and what is a function?
A formula starts with = and calculates something. A function, like SUM, is a built-in formula.
How do you sort and filter data?
Use the Data tab: Sort to order the rows, and Filter (Ctrl + Shift + L) to show only the rows you want.
What does AutoSum do?
It inserts a SUM formula for the cells next to it. The shortcut is Alt + =.
Intermediate questions
Difference between COUNT and COUNTA?
COUNT counts numbers only. COUNTA counts all non-empty cells.
What is VLOOKUP?
It searches the first column of a table for a value and returns a value from another column in the same row.
What is a PivotTable?
A tool that summarises a large table in seconds by dragging fields.
What are absolute references?
References like $A$1 that do not change when a formula is copied.
Practise each answer in a real spreadsheet. It is much easier to explain something you have done.