how to make giveaway ?

Yes, you do!
Click on "Giveways" tab in the left upper corner and select "Create a New Giveaway". Type the game name and select the right one. (If you can't find it, don't make it for another game!) Select if you gifting a key or a steam gift and, if it's a key, type in the key (you can also choose to select "gift" if you want to add the key after the giveaway ends, instead). Choose the start and end times, the appropriate region lock if any and if you want it to be public, private or group/whitelist. Select a level and add a description if you wish. Click "review giveaway", review it and then confirm, if everything is ok. Remember that you will have only 3 slots at the beginning and deleting a giveaway makes you lose one. If you lose all the slots, you will have to contact support and ask for more, so better just be careful.

^ Up there on the left, the button that says "Giveaways" on it. Click the little arrow on that button and select "Create a New Giveaway."

You press the down symbol next to "Giveaways" and "Create a new Giveawa"