How to customize Check Out form, process & design

How to customize Check Out form, process & design

Hi

Just set up my first shop and changed the design quite a bit. Like it so far! However, did not manage to change either Design, nor Content, nor Process of the Check Out. Where can I change this? E.g.
- change the font of the check out forms
- change the design / elements of the check out pages
- change the attributes in the check out form

Thx
Thomas