How to have a star rating field on custom object in salesforce

Well, in that case, you'll have to create a separate field, most probably a Picklist Field would suit the need. You can create a Picklist Field say -
Label: RatingCode
Type: Picklist
Values:
1
2
3
4
5
And then create a another Formula Field on the same object. This is to represent the Rating "pictorially". That field would look like this -
Formula Return Type: Text
Label: StarRating
Type: Formula

Formula: 
IF(
    ISBLANK(TEXT(RatingCode__c)),
    NULL,
    IMAGE(
        "/img/samples/stars_" + TEXT(RatingCode__c) + "00.gif",
        "Rating Level"
    )
)

Output:-