Check out the links I posted in the answer to this question for some links to SharePoint 101 tutorials online:

Restaurant review site in SharePoint

Given you re looking for branding that will mean a custom Master Page. A Master Page is what will determine the overall layout of your SharePoint Web Application s pages. After creating a standard SharePoint application, connect with SharePoint Designer and play with that Master Page for a while until you get the hang of it.

In modifying or creating custom Master Pages, it is important to take note of all the controls (special ASP.NET server-side tags) in particular Content Place Holders; if you create your own Master Page and these are missing you re in a world of uninformative-error-message hurt.

You re going to want to create Lists ( http://office.microsoft.com/en-us/sharepointtechnology/HA011199881033.aspx ) to represent News and Products ... for Products you may even want to create a Content Type.

Once you create News and Products lists, to see those lists of Products you ll want to make views and use List View Web Parts ( http://office.microsoft.com/en-us/help/HA100240521033.aspx ) to include those views in Pages... but at the subject of Web Parts this is probably getting a bit overwhelming so I ll stop here. Comment me if you d like more.

Addendum

"Is is possible to show only categories of a document library in home page and documents of the selected category in another page?"
-> Yes, and there are several ways. I d suggest you look into either defining a View for that list which filters based on the category field, then add List View Web Parts to your home page, or writing custom XSLT for the Content Query Web Part, and then add it to your home page. Writing the XSLT for he Content Query web part is a little more difficult, but using the List View has it s short comings in being not-as-configurable, much like this guy is finding out: No-code solution for calendar view of SharePoint news items

"But how can i change a list item to show its details as a whole new page with its own web parts and theme?"
-> Here s the kicker; in SharePoint, each Site you create has a special list for Pages, and each Page actually belongs to a Content Type. So you can take any content type and turn it into a page; all you have to do is create a Page Layout for it, and add that Content Type to the list.

(So, an example. This is purely speculative; I m not saying do this specifically, and you probably can t do it exactly as I describe it as I m oversimplifying a little)

You create a Content Type Product , but you also decide Category should be a Content Type too. So you create a content type for Catagory , and you give it a lookup column, which refers to items in the Product List. You specify that the lookup can contain multiple items.

Now you want a Page where people can see a category, and all the Products which belong to it. You create a Page Layout (an aspx file) for the Catagory Content Type. In the Page Layout aspx file you can include web controls which render the Catagory s fields, such as title and description and you can probably even add one to render the Products lookup field. (maybe you ll have to code it yourself)

I hope I haven t lost you.

Now that you ve created a Page Layout for Catagory you can add Catagory to the Pages list/library and then create pages which represent Catagories, and when people edit those pages, they will also edit the Catagory.

More Addendum

If "Announcements" is a list, the web part you added was a list view web part, and so it just shows the contents of the list (which is independent of the product pages) You ll want o specify filters of your view. I dunno if you can specify unique filters on the web part... but you can try :)

You might want to consider looking into The Content Query Web Part.

I don t fully understand your second question, but I get the feeling the answer is yes... what do you mean by "view"? You gotta be careful with the lingo; do you mean the Page Layout? or the List View Web Part? or the List View? or the Master Page? ...

you HAVE to check out Heather Solomon. She is the shiznat when it comes to sharepoint branding.

A good start would be using Sharepoint Designer.

Use SharePoint Designer to do your CSS work, particularly in discovering the CSS rules in play. DO NOT publish the finished work from SD; if you do, future Windows Updates may wipe out your files. Instead, deploy your customizations as a Feature. In MOSS 2007, you can add a single custom CSS file using Central Admin - this will then be applied AFTER core.CSS at runtime, To the best of my knowledge, this is the only practical way of doing this.

You need to indicate whether you are using WSS or MOSS. With WSS, you can use themes.