Every provider (AT&T, Comcast, Verizon, etc.) makes their own choices of what kind of modem to use and how to set it up for new users. Generally speaking, a provider is going to give you the cheapest modem and the easiest setup to make setup cheap and easy for new customers.
I have AT&T DSL service. It uses a modem called 2Wire which includes Wifi. While I can only speak to AT&T's setup of the 2Wire box, regardless of your provider you should check to make sure that you are not affected by the same giant security hole that AT&T sets up.
AT&T sets up your Wifi to be called 2WIRE + 3 random digits. Click the Airport icon on your Menu Bar and you will see all of the Wifi networks in range. You will probably see a few networks called 2WIRE. At the coffee shop where I am working, there are four. This is AT&T's way of screaming "Hey, look at me! I am easy to break in to!"
The problem is that AT&T's easy setup configures the Wifi to use an old form of encryption called WEP. WEP's encryption is so bad that is is generally regarded as no encryption at all. In fact, I watched a video on YouTube that demonstrated cracking a WEP protected Wifi network in seven minutes.
The modern encryption standard for Wifi is called WPA. That is what you should be using.
Checking the encryption type of your Wifi network is very easy: Hold down the Option key and click on the Airport icon on the Menu Bar. You will see detailed information about your Wifi network. Four lines down is Security. It should say WPA Personal. If it says "WEP Password", you need to change it right away.
(For fun, select one of the 2WIRE networks on your list on the Airport icon. A dialog box will pop up asking for either a WEP or WPA password. Chances are very good the 2WIRE ones will ask for a WEP password and you are only seven minutes from accessing that network. All four of the 2WIRE networks near me are using WEP encryption.)
So what do you need to do?
- Change from WEP to WPA encryption.
- Change the name of your Wifi network.
My best advice is to call your internet provider's technical help line and tell them you want to change these two settings. They will be able to walk you through the steps.