There are two ways to do this. Either using the Alter.Table expression or the Create.Column expression.
Here is an example from the wiki for Alter.Table:
Alter.Table("Bar")
.AddColumn("SomeDate")
.AsDateTime()
.Nullable();
And here is an example of Create.Column:
Create.Column("avatar_name")
.OnTable("user")
.AsString()
.Nullable();
Generally with FluentMigrator you can lean on Intellisense to help you out. For example, if you start by typing in Create and dot then Intellisense should show you Column, ForeignKey, Index, PrimaryKey, Schema, Sequence, Table and UniqueConstraint. Most of the time this process should be very discoverable and obvious.
There are seven roots that you can start with: Create, Alter, Delete, Execute, Rename, Insert and Update.