am i supposed to connect to the internet through command line or by some GUI program?
You don't connect to the internet. You connect to your router and the router connects to the internet. The router (assuming has dhcp turned on) assigns you an IP address. You should let the slackware wirless scripts set all the iwconfig parameters and run a program to obtain an IP from the dhcp server.
You will learn more about the whole process if you use the command line and scripts in /etc/rc.d/ I personally haven't used GUIs to set up my wirless or network.
You should start with the manual pages for all the tools involved. Also, it helps if you can follow the scripts slackware uses to understand how it is doing everything to get you connected.
(also check the manuals for commands listed at the bottom of those pages under SEE ALSO)
Those 3 tools require root permissions for the settings to change. e.g.
su -c '/sbin/ifconfig wlan0 up'
assuming you have the correct driver/firmware installed; after a reboot, the module will be listed with /sbin/lsmod
/sbin/lsmod | grep b43
IF your wireless router is properly configured to assign ips AND it is the only one in your area. Slackware will automatically connect to it on boot.
You can force it to connect to only your network by editing /etc/rc.d/rc.inet1.conf (read the comments in the file).
ALSO! linux is not windows! iwconfig does not translate text string WEP keys into hexadecimal (0-9 and A-F) If you really must attempt all this with WEP turned on, make sure you use just hexadecimal. OR if you know what you are doing, you can convert a text string from ascii into hex and place that in the /etc/rc.d/rc.inet1.conf file. Until you get this figured out, I would attempt all this with WEP turned off.
The reason I don't give you step by step is because you said:
I chose Slackware cause i was told it was the most difficult distro.
This tells me you want to do it on your own. Keep in mind, wireless was one of the more annoying things for me to get configured on my system. Just keep at it and keep reading everything.
What I would start with to just see if everything works.
Assuming your access point is properly configured to assign ips and does not have the wep turned on...
uncomment the following variables and edit them
change the host name to the ip of your router (generally 192.168.0.1)
change the gateway (in the same file) to the ip of your router
change the essid to what you set it to on the router
edit any other parameters you may need.
... sorry but I forgot if the hostname and gateway actually need to be filled in. If you can connect over cable and the eth0 section does not have an ip listed, then you probably don't need it for the wlan0 section.
after doing that.
unplug any ethernet cables from your computer and restart your computer. (a computer restart may not be required BUT as I said, I had issues with the b43legacy drivers not connecting at any time other than boot.)
IF all that works, then you know you have the ability to get connected with your wireless card. I would continue reading about the commands and playing around with scanning and other settings.
If that doesn't work and b43 is being loaded, you may have to use ndiswrapper anyway.
The scripts that slackware uses in /etc/rc.d/ are generally specific to slackware but are far easier to follow than in other distros.