I got a tomtom. How do I make it water proof?

Regular GPSs ARE waterproof. You have a navigation box that uses GPS.

I dunno, I'd guess plastic bag, if there aren't any commercial products specifically for it.

Regular GPS? Tom Tom?

What type is it? My assumption is a regular car navigator or motorcycle type of GPS. If I'm not mistaken, I don't think TomTom make handheld unit like Garmin or Megellan.

If that is true, then the motorcycle unit are already water proof. You can try using a map bag that strap on to your handle bar and put the unit it that. The bag has a clear plastic face for placing a map and since most GPS are pressure touch screen, you can still activate the menu.
Here is a link from REI
https://www.rei.com/product/780457

Your other choice is to have a place like Tap Plastic here in the west coast or whoever acrylic store in your area to custom make a 1/8" Acrylic box for your unit. You can make it with six side but leave one side (left or right) that has a slot so that you can put theunit in it or your hand in it to select your menu.

Otter box if they make one for it.

Could I just put tape over the power button and speaker then plug up the usb port?

Ziploc Freezer bag - many sizes to choose from.

This is the cheapest and easiest way.

stretch a condom over it and tie a knot.

I've been using a Garmin eMap as my cyclometer for over 10 years now. It is also not at all waterproof. But it fits perfectly in one of the small 'snack-size' ziplock bags (about half the size of a regular sandwich bag). The handlebar mounting hardware still works fine with the GPS inside the bag - the screw holding it on pokes through the bag but is tightly sealed against the mount so there is no possibility of leakage there. The TomTom models are a little bigger so the regular sandwich bag would probably be more suitable.