Hello,

You can begin your speech in this way:

On the behalf of the management, I welcome you as a new addition to our company. Since this is your first day, I wish you all the best for the new beginnings. We all look forward to work with you. You must not be familar with the opeartions so you can approach to the seniors and collegues anytime for your doubts. Rest, we are confident you will never let us down and prove to be an asset to our firm. Welcome aboard!

Hope it helps.

Good Luck!