I want to buy a wireless lan for my PC in order to connect to the internet through an ISP, I wan you help me to find a good one! In Fact I know a little bit about these devices but I haven't had one before so I'm a novice and don't know too much about Wlans. Is there an EXPERT in wireless techknologies here?

To setup a WLAN that will connect to the ISP/Internet - what you need is an AP and Router combo - typically referred to as a Broadband Router / Gateway device - a typical device is Linksys WRT54G. So what happens is that your PCs connected "wirelessly" to the AP that is present in this device and then WAN port in this device is connected to the ISP's modem. If you are using Cable Modem there are some models that have the Cable Modem built-in. I would recommend a device that does not have modem - so I will suggest WRT54G or a similar device.
If you are going to play games or video-stream inside the home heavily I would suggest that you recommend either a 802.11a or 802.11N router. If it is just 'typical' internet usage emails - downloads movies then 802.11g in a device like WRT54G is sufficient. Hope this helps... any questions? I would recommend using 802.11g or 802.11n devices because they operate in ISM band - what that means is it that broadly speaking there are no restriction (world-wide) on their usage, though there could be channel restrictions. Please check with the local seller for the legal/permitted channels of usage.

What is your PC capable of in terms of wireless, an 802.11? , What are the technologies for the "?", b, g, n?

What is your internet? DSL, Cable modem?

What kind of structure: House, apt? # of floors?

To clarify some deinitions:
AP = access point or what device is delivering the wireless signal

I got a simple stupid PC! In fact I know about wlans and I have the informations about wlans, how do they work or what do they need, my problem is that my informations about their models and usages are not complete! For example I have got some barriers in front of my home or between the home and that certain AP, I'm looking for a good one which is powerful and can work properly without any noises despite the barriers and costing little money?
And I don't want to play online gaimes! I just want to use it for downloading files, checking mails, again downloading files :D! (I'm a download junky).
theraos what do you recommend? Please write down ful name and its model. THANK YOU SO MUCH theraos! :-)

As KISS was mentioning, you want to match what your computer is capable of to the wireless access point that you choose (A, B, G or comibination thereof). More importantly, you should configure the security in it so that others can not use it without your knowledge or permission.

Also, keep in mind that your Internet connection is the weekest link. Even the slowest of wireless connections will give you 12MB/sec speeds whereas your Internet connection will generally give you up to 4MB/sec speeds for download and maybe 512KB/sec to 1MB/sec speeds for upload.

If you intend on using your wireless network for file sharing and printing, then the faster 54MB/sec wireless access points will be more to your liking.