At the end of the line of text or heading you have typed press the ‘shift carriage return’ key on your keyboard, this just moves the cursor to the next line, rather creating a new paragraph or double line spacing.
To remove an unwanted space above a heading, paragraph or sentence you need to get the cursor where you want to remove the space so click the area with the left mouse key so it knows where to put the cursor then click the Little T icon which you will see along the top of the screen along the text editor options (the one 2 back from the binoculars symbol) - as shown below in red:
When the cursor is then shown you can use the backspace key to get rid of the top space.