Creating a standard playlist is pretty straightforward. You just hit the plus button in the bottom left corner of iTunes, or go to the file menu and select New Playlist. iTunes will create an "untitled playlist" that you can name whatever you want.
To put the songs on your playlist in a certain order, you can sort by any of the column headers. If the default columns don't do it for you, there's many more column fields under View Options. To tweak the playback order on a song-by-song basis, sort by the first column, which lists songs numerically, and drag songs up and down the list until you're satisfied.
Another playlist trick you can use are these check boxes next to each song. By unchecking a song, iTunes will skip over the track, but keep the song in the playlist. This comes in handy if you have something like a dinner music playlist you use regularly, but maybe tonight's guests can't stand hearing Enya, so you just temporarily uncheck your Enya tracks from the mix.
Now, if you start going crazy with playlists and make dozens of them, you can also play around with making playlist folders. This is like a playlist for your playlists, and helps keep things organized. To make a playlist folder, go to the file menu and select "New Playlist folder." Name it whatever you like and then start dragging playlists into the folder. You can collapse and reveal folder contents using the arrow to the left of the folder.