Once that I created the scaffold, how can I change "title:string" and "description:text" ?

I found this information here, I was curious about the same thing!

http://guides.railsgirls.com/guide-to-the-guide/

Hopefully, you've understood a little bit about models, and how they are meant to act as data encapsulators. Happy to share it! Here's what you're looking for-

a model can have attributes (properties) represented by columns in the corresponding database table. To be supported by the Active Record system, these attributes must conform to a list of appropriate types:

:binary - stores data such as images, audio files or movies

:boolean - stores true or false values (such as whether a particular user is an administrator of an application or not)

:date - stores only a date (year, month, day)

:datetime - stores both a date and a time

:decimal - stores decimal numbers with precision that varies according to your specifications

:float - stores decimal points with fixed precision i.e. you can’t specify the precision (:decimal is better for mathematical operations in which precision is required, but :float is processed faster and is better in situations where speed is required and accuracy is secondary)

:integer - stores whole numbers

:primary_key - the primary key of a table is assumed to be the id

:string - stores 255 characters of text information, i.e. is used for short text fields (names, emails etc)

:text - stores text information with no character limit (used for comments, blog posts etc)

:time - stores only a time

Listen to paragraph :timestamp - stores both a time and date. :timestamp is different from :datetime and serves a different purpose, but there’s no need to go into that here