Hi,

Create sequence for your object Go Settings >> Sequences & Identifiers >> Sequences

Add new file yourfilename_sequence.xml in your module pase this code ans change as per your requirement, add in __opnerp__.py update module

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data noupdate="1">

        <!-- Sequences for sale.order -->
        <record id="seq_type_sale_order" model="ir.sequence.type">
            <field name="name">Sales Order</field>
            <field name="code">sale.order</field>
        </record>

        <record id="seq_sale_order" model="ir.sequence">
            <field name="name">Sales Order</field>
            <field name="code">sale.order</field>
            <field name="prefix">SO</field>
            <field name="padding">3</field>
            <field name="company_id" eval="False"/>
        </record>

    </data>
</openerp>

Thanks