Many cellular providers (Verizon is one notable exception) block the use of Skype and many VoIP services on their data networks. In these cases, you will only be able to use Skype if your phone has the ability to access a Wi-Fi connection, which is generally much faster anyway.
I have used Skype on my Android using T-Mobile and the results are not very consistent. I have not been able to use it at all on T-Mobile's 3G network. I suspect that they are blocking Skype's traffic. It works fine most times using a Wi-Fi link to my DSL connection at home.
The two biggest issues that will affect the quality of your Skype calls on wireless are connection speed and latency. Most VoIP calls use between 64 and 90 KBps in bandwidth. So, make sure your data connection is MUCH faster than that. Latency can be measured using a PING utility, which is available on some smartphones. Unfortunately, other than upgrading your own Wi-Fi connection, there isn't much you can personally do to improve available bandwidth or latency issues. Try to find a mobile provider that has the fastest speed and lowest latency if you want your Skype calls to sound good though.