When your typing creates a new cell in a notebook, its typically an Input cell. You can change the default to a different style.

Interactively...

Open the Option Inspector

Choose Format Option Inspector () to open the Option Inspector:

Select the notebook whose new cell default style you want to set

Select the notebook whose new cell default style you want to set from the popup menu at the upper left:

Locate the DefaultNewCellStyle option

Type DefaultNewCellStyle (without quotes) in the Search field to locate that option:

Set DefaultNewCellStyle to the style for new cells

Select the style you want to be used for new cells from the menu, or click the style name and type a new name (with quotes):

Create cells in the new style

Newly created cells are now Text cells:

Programmatically in the Current Notebook...

Set a notebooks DefaultNewCellStyle option to specify what style to use for new cells:

Programmatically in a Newly Created Notebook...

Use the DefaultNewCellStyle option in a newly created notebook to specify what style to use for new cells: