plsql if control different calculation

I am using PLSQL to realize some of the function below.

I have the table which have piece level data with each piece weight. Basically I want to realize the following function:

1. if piece weight is over 1 LB. groupby ceil(weight) (next LB)
2. if piece weight is less 1 LB groupby cell(weight*16) ( Next OZ)

I am just curious how can I realize that in plsql. I feel I need to have the if statement. But I am not sure how to do that.

(Weight is already an variable in that table, do I need to declare here?)

        begin
            if weight <1 then
              select ceil(weight*16),sum(weight)
            from ops_owner.track_mail_item
            where manifestdate = '24-aug-2016'
            group by ceil(weight*16)

            else select ceil(weight),sum(weight)
            from ops_owner.track_mail_item
            where manifestdate = '24-aug-2016'
            end if,
            end;

Thank you very much!