Display entry category
To display a single category you'll need to do something like this:
{% set category = entry.categoryfieldhandle.one() %}
{{category.title}}

To display multiple categories:
{% set categories = entry.categoryfieldhandle.all() %}
{% for category in categories %}
{{category.title}}
{% endfor %}

Replace categoryfieldhandle with the handle you used for your field.
You can find more in depth and alternative ways in the docs:
https://craftcms.com/docs/3.x/relations.html
Since you're new to craft I recommend taking a look at the following course https://craftquest.io/courses/craft-cms-3-tutorials to help get you started.