For which property are you trying to set the defualt value? Is it CompanyName? If yes, you can set it a default value in the constructor or at the time you declare this variable.

public class Company
{
public String ETag {get; set;}
public String Id{get; set;}
public Link[] Links{get; set;}
Public String Address{get; set;}
Public AddressParsed AddressParsed{get; set;}
public String City{get; set;}
public String YearEstablished{get; set;}
public string BusinessStatus {get; set;}

// [DefaultValue('Not Available')]
public String CompanyName{get; set;}
/* public String CompanyName
{

get
{
CompanyName = 'Not Available';
return CompanyName;
}
set
{
// if(CompanyName == null)
// {
// CompanyName = 'Not Available';
// }
// else
// {
//CompanyName = CompanyName;
//}
}
}*/
public String FirstName{get; set;}
public String LastName{get; set;}
public Location Location{get; set;}
public String Phone{get; set;}
public String PostalCode{get; set;}
public String Website{get; set;}
public String StateProvince{get; set;}
public c_Contact[] Contacts{get; set;}
public String Neighborhood{get; set;}

public Integer CorporateEmployeesSizeActual{get; set;}
public String CorporateEmployeesSizeRange{get; set;}
//public Integer CorporateSalesVolumeActual{get; set;}
public Decimal CorporateSalesVolumeActual{get; set;}
public String CorporateSalesVolumeRange{get; set;}
public String County{get; set;}

public String CreditRatingScore{get; set;}

public String EIN1{get; set;}
public String EIN2{get; set;}
public String EIN3{get; set;}
public String Fax{get; set;}
public String Gender{get; set;}
public Integer LocationEmployeesSizeActual{get; set;}
public String LocationEmployeesSizeRange{get; set;}
public Naics[] NaicsList{get; set;}
public String PrimaryNaics{get; set;}
public String PrimaryNaicsDescription{get; set;}
public String PrimarySic{get; set;}
public String PrimarySicDescription{get; set;}
public Sic[] SicList{get; set;}
public String Title{get; set;}
public String TollFree{get; set;}
public UriCls[] UriList{get; set;}
public String ZipPlus4{get; set;}
public String BusinessType{get; set;}

//Additional Property for Companies API

public String Franchise{get; set;}
public String LocationSalesVolumeActual{get; set;}
public String LocationSalesVolumeRange{get; set;}
public String TickerSymbol{get; set;}

//Additional Property for Match API
public String RuleName{get; set;}
public Integer Score{get; set;}
public String IsHomeBusiness{get; set;}

public EmailVendor emailVendor {get; set;}


public Company()
{
this.Links = new List<Link>();
this.CompanyName = 'Not Available';
this.CreditRatingScore = 'Not Available';
this.TickerSymbol = 'Not Available';
}

}

above is my code i assign value in constructor but its not work.

i want to dispaly default value if there is null value.
if company doest have any value then its display 'Not Available'.