I attempted my first fondant tiered cake a few months back and at first it looked really good until the top started to sink into the bottom.
Two of my friends have asked me to make their babyshower cakes. Atleast 2 tiers.
What is the best mix to use for the cakes?
What can be used to support the top tier to prevent it from caving in to the bottom?
Thank you in advance for any help.

Dowels, straws, SPS, something. You gotta use something, or it will sink. Go to Wilton's website and find the area about supporting cake tiers. That's a good place for a start.

Hi and Welcome to CC, tsc1220.

Everything you need to know to make, decorate and assemble tiered/stacked/layer cakes:

http://www.cakecentral.com/cake-decorating-ftopict-605188-.html

Above super thread has popular CC recipes for crusting American buttercreams, several types of fondant and doctored cake mix (WASC and other flavor variations) and so much more.

The more consists of cake making help from Wilton, and hints & tips from CC members. There's also a link to photos of common cake support systems with complete and accurate directions. Also see leah_s info on SPS.

In addition to cake and frosting recipes, there's also help on smoothing your frosting using various methods.

Get comfy, it's a long read.

HTH

Hi and Welcome to CC, tsc1220.

Everything you need to know to make, decorate and assemble tiered/stacked/layer cakes:

http://www.cakecentral.com/cake-decorating-ftopict-605188-.html

Above super thread has popular CC recipes for crusting American buttercreams, several types of fondant and doctored cake mix (WASC and other flavor variations) and so much more.

The more consists of cake making help from Wilton, and hints & tips from CC members. There's also a link to photos of common cake support systems with complete and accurate directions. Also see leah_s info on SPS.

In addition to cake and frosting recipes, there's also help on smoothing your frosting using various methods.

Get comfy, it's a long read.

HTH

Welcome to CC! Jan just gave you the wealth of knowledge you will need. That being said, as you begin the process don't hesitate to ask additional questions. Someone is always willing to try to help you out.

REMEMBER THIS ALWAYS: Have fun, it's just cake!