How can I see the templates to choose suitable a template for my store ... and how to modify it?

You can go from the control panel to the design then the templates and inside it will show you all the templates and you can sort these templates according to your product's identity, whether it's business, technical, sports … etc,

You can click Apply Template after you have settled the template to fit you. The following images are for your reference

You can modify the template by going to the Customize Template tab, 

You can see also the following videos that were created to help you design the store

https://youtu.be/e2XbeAq2xJg

https://youtu.be/qFFT-AZMMWQ

https://youtu.be/bynkPEUAoK4