how to uniformly select rows in sql
You can use row_number and the modulo operator

Tables don't have any inherent order. You need to define an ordering expression (e.g column names) to define what "first" and "next" mean.

DECLARE @StepSize int = 2;

WITH CTE AS
(
SELECT *,
       ROW_NUMBER() OVER (ORDER BY YourOrderingColumn) AS RN
FROM YourTable
)
SELECT *
FROM CTE
WHERE RN % @StepSize = 1;