If I want all the slide in the presentation to have the same “look”, what should I do?

You can use duplicate option , or when you finish you can go the design and choose which one you want and make apply to all slides by right click on it

Go to Slide Master and make the changes. This format will reflect in all the slides. 

For Example if you want your companies logo on all the slides. just put that in the slide master and you are done.

make a master slide for the presentation and add contents to you slide

I use Microsoft PowerPoint2010,

1) Go to the design tab

2) Right click on the design of your chocie

3) Then click apply to all slides

once you select your look from design section you don't have to do any thing else because once you make a new slide it will look like the one befor