how to upload files using Apex code in Salesforce?

The easiest way to do this is by using inputFile and assign it to the ContentVersion instance. Like this:

<apex:page controller="ContentController">
<apex:form>
    <apex:inputFile value="{!file}" />
    <apex:commandbutton action="{!upload}" value="Upload" />
</apex:form>
</apex:page>

Class:

public class ContentController {
    public blob file { get; set; }

    public PageReference upload() {
        ContentVersion v = new ContentVersion();
        v.versionData = file;
        v.title = 'testing upload';
        v.pathOnClient ='/somepath.txt';
        insert v;
        return new PageReference('/' + v.id);
    }
}

If you want to share the ContentVersion file then after inserting DML query the ContentDocumentId from inserted ContentVersion and use ContentDocumentLink to create an association between Record and ContentVersion uploaded a file.