My first wedding cake, the topper is heavy, the one from Walmart, the top tier is very small and I can picture it falling over and sliding down the cake!
Any Advice or tips?

You need to support it just like you would for your tiers. I personally use straws because they are easy to cut and hold up well. But whatever you use, cut them to the length you need, then put a cardboard circle (cut to fit the topper so the cardboard doesn't show) on top. Then just set the topper on the cardboard circle.

I had the same problem with one of the toppers in my Double Wedding photo. The topper was really tall and made of ceramic with a wooden base! The ceramic figures weren't connected in any way to the base. so I had to run out and get some sticky tack to keep it from falling off! I used several wooden dowels under the wooden base to keep it from sinking into the cake. I wasn't at the wedding, but a good friend was. and to play a joke she called me halfway into the reception to tell me the cake had been knocked over by a kid running into the table! Needless to say I didn't think it was very funny!

It will be fine as long as you support it well enough. Good Luck!

Thanks ya'll! That will be much better.
Wish me luck