While different Content Types will have different options, the will all require you to enter a Title, and most will have a Body where you put the main content.
The Body section will most often have a 'wysiwyg' editor (what you see is what you get) which will provide a range of 'buttons' that will help you format the content without needing to learn html.
The editor will probably look something like this.
Many of the buttons are self explanatory, but there are some important ones to remember.
Pasting from Word and other documents
When you paste something from another document such as a word document, there is a risk that you will be pasting in formatting code that does not work well on web pages. Therefore there are two buttons that can help.
One has a T for Text, and one has a W for Word. If you have a document that you have formatted with headings etc in Word, then you can use the 'Paste from Word' button, otherwise it is generally safer to use 'Paste as Plain Text' and use the wysiwyg editor to apply correct html formatting.
Adding Hyperlinks and Anchors
To add a hyperlink to some text in order to make it link to another web location, first enter the text you want people to see, then click on the 'chain' button and enter the destination URL in the Link URL field. You can also set whether you want the new page to open in an new window or in the current one. If you need to change a URL simply click any where within the hyperlink and the 'anchor' will stop being greyed out, and you can click on it to edit the existing settings.
To set an anchor so that you can link to somewhere within the same page or a specific section on another page, click on the anchor symbol.
Bulleted and other lists
You can set up numbered or bulleted lists using the 'unordered' and 'ordered' lists buttons.
Teaser
One element that is likely to be new to you is the notion of setting up a 'teaser' for an item. The purpose of these is so that if the content of the page is shown on a summary page, then you get to determine how much of the text should show as the 'teaser' So if you want to override the default option, use the 'separate teaser' button.