How can I don't let people to know when is the last time I sign in to my whatsapp .I just want to hide my signing in to whatsapp and I don't want my contact to know that I'm online at this moment?

To hide your last seen status and online presence on WhatsApp, go to Settings > Privacy. From there, you can select Last Seen and set it to Nobody or My Contacts (if you want only select contacts to see it). Additionally, you can disable the Read Receipts option to prevent others from seeing when you've read their messages. To remain completely hidden while using the app, consider using Airplane Mode to read messages without appearing online, but remember to reconnect to the internet afterward to send messages.