There are so many different things you need to find out, when you move into fountain pens, and there are so many different pens to pick from. But I think there are two pens you should get to start out with. Both of them are cheap: The Lamy Safari and The Pilot Metropolitan.
I think the two of them are good for very different reasons. The Pilot is a overall very well built and fantastic pen, and The Lamy Safari have a much better nib. The reason I think you should get both is to find out what you prefer. The big problem with the Safari is the grip section, it isn't that good for left handed writers.
But I think the Lamy Safari is a very good pen to use to figure out what kind of nibs you like.
Go for a medium nib, unless you already have a strong preference. The journey starts with your first fountain pen. I like a smooth and very wet nib that leans a wide line, while others like a more scratchy and narrow line, that enables them to write tiny. It all depends on your preference.
Most of you do probably already have a preference when it comes to width, are you buying the 0.7, 0.5 or are you one of the crazy 0.3 people?
There is only one way to find it out, and it will probably change over time.
Let's get back to my point. I think it is good to get both pens to experience two very different pens, from the two biggest names in fountain pens. I still use both of them, and it was with my Lamy Safari that I tried many different nibs to figure out what I preferred.