If you made the HSA contribution by having your employer take it out of your pay, it will be shown in box 12 of your W-2 with code W. You just enter it in box 12 when you enter your W-2.
If you made a contribution directly to your HSA yourself, not through your employer, go to the "1099-SA, HSA, MSA" topic in TurboTax. After it asks about expenses you paid from your HSA and whether you inherited the HSA you will come to a screen that asks if you put money into an HSA. If you answer Yes you will get a screen where you can enter the contribution that you made.