Try out the sample code given below :

VF code:

                         <apex:form >

                                <apex:pageBlock title="Upload New Attachment">

                                <table>

                                   <tr>

                                       <td class="rows" >Browse For File</td>

                                        <td><apex:inputFile value="{!filebody}" fileName="{!filename}"  /></td>

                                    </tr>

                                     <tr>

                                     <td></td>

                                      <td align="left"><apex:commandButton action="{!uploadfile}" value="Upload"/></td>

                                    </tr>

                                   </table>

                               </apex:pageBlock>

                          </apex:form>

Controller code:

                                                public string parentid { get; set; }

                                                public string filename { get; set; }

                                                public blob   filebody { get; set; }

                                                public void uploadfile()

                                                {

                                                                if(filebody != null && filename != null && filename != '')

                                                                {

                                                                                transient Attachment a = new Attachment();

                                                                                a.body = filebody;

                                                                                a.name = filename;

                                                                                a.parentid = ApexPages.currentPage().getParameters().get('id');

                                                                                try

                                                                                {

                                                                                                insert a;

                                                                                                filename = '';

                                                                                                filebody = null;

                                                                                }

                                                                                catch(Exception e)

                                                                                {

                                                                                }

                                                                }

                                                }

Hope this helps.