and as you say you have a limited knowledge of web design I think the first thing to do is to identify what you want your intranet to do, and then we'll decide how to do it.
Broadly speaking an intranet can be likened to a web site that runs in a browser, and can only be accessed by company employees. Those people don't all have to be in one place, they can be at different locations in the same office complex, in the same country, or in different countries - all of them can access the intranet, but other people can't.
If you want authorised 'strangers' to have access to your resources, or to limited areas of them you can do it, but then you'll be designing an extranet - accessible to people outside the company, but on a limited basis. The general public still won't be able to see your content.
A simple intranet site consists of a number of pages with relevant content, and links to available resources, like shared documents and so on. You organise the documents in folders within the web and list them in some appropriate fashion on your pages. The links in the listings allow users to open the documents on their desktops, but not to edit them. Your intranet can also link your users out to the internet, and of course the whole thing runs in the users' browsers.
For a simple intranet like this you can use normal web-design software like MS FrontPage or NetObjects Fusion, but if you want anything more sophisticated you'll need to consider one of the many specialist intranet software solutions that are on the market.
It might be worth considering some of them before you make a decision. These offer all kinds of facilities like messaging, email, document tracking etc., and depending on your requirements and your budget there's no limit to how sophisticated your intranet can be.
click here to get a better idea of what can be done, and what's available to do it with. If that still leaves you feeling a little unsure please don't hesitate to come back and ask for some specific advice. I have designed quite a few intranets for clients - from simple web sites running internally to big intranets for multinational organisations with lots of locations in several countries - and I've had some experience of the pitfalls (learnt the hard way!).