Which lens should I look at purchasing?

I have the Nikon D90, with the 18-105mm F/3.5-5.6 lens.
Now that I've learned how to shoot in manual mode, I'm looking to get a lens that's better with shallow depth of field, as well as a macro or maybe a filter for macro?
I see a lot of lenses with a small f stop number, but they're not zoom lenses as far as I can tell, and since that's what I'm used to, I'm a little confused at how it's going to work.
So. Which lens for dop would be best?
Which lens for macro? Or should I just get a filter?
And if it's not a zoom lens, what do I need to know?

Added (1). I need the macro because I like taking close ups of flowers and such.
The lens for shallow dop is for portraits.

The widest aperture zoom lenses are F/2.8 and they are expensive!

To get your shallow depth of field then a 50mm f/1.4 should do the job.

For macro then you can add lenses tothe front of the 18-55 but it won't help a great deal. I use a macro lens on the filter threqad of a 55-250 and at 250mm I get reasonable results. At 55mm I can hardly tell the difference between that and a shot at 250mm without the filter and further away.

Best macro is from extension tubes of a dedicated macro lens - a 100mmm F/2.8 is good.

Okay, you need to read up on different lens types…

then you can understand:
prime
macro
telephoto
zoom
wide angle
ultra wide angle
fisheye

then you might learn about what each is capable of…

what are you needing a shallow depth of field for? Portraits? Insects/flowers?

generally, macro photographers don't use a shallow depth of field because it can be TOO shallow…

once you know what you are shooting, THEN you can decide on the lens…

[edit - if you are wanting one lens - there are 2 options open to you:
1) a 50mm f/1.4 prime lens (which is the current standard for portrait photography, if you aren't very imaginative) - and use a set of bellows/extension tubes for the macro aspect… Though try to be creative and avoid the cliched use of flowers and insects… Or
2) a 50mm f/2.8 macro lens - though you would have to be very close to the subject of the macro… ]

1- don't use macro "filters". They are garbage.

2- In Canon I would recommend the 24-70 f2.8 (1600$) as it has decent macro setting and is a good portrait/landscape/event lens but I'm not sure if the Nikon equivalent is the same (in terms of Macro capabilities).

3- If you get a PRIME lens (one that doesn't zoom) then you need to know that you zoom in or out by phisically moving forwards or backwards. This is fine if you have alot of space but can be limiting sometimes. Of course, prime lenses are typically much cheaper than zooms (simpler construction) and USUALLY have better image quality (less tradeoffs made during design).

The real question is… What is your budget. I've shot with 120$ lenses (Canon 50nmm f1.8) and 14000$ lenses (Canon 800mm f5.6 L) and everything in between.

This could be a nice choice for you:
NIKON AF-S 60MM/2.8 MICRO at 580$
http://www.henrys.com/853-NIKON-AF-S-60MM-2-8-MICRO.aspx

But I haven;t read any reviews on it (I'm a Canon guy).

What's your budget?

I've never used a macro lens I didn't like (but some are better than others). These days I often use a Canon MP-E65 lens, but I'm afraid Nikon doesn't have an equivalent to that one. If you're going to take a lot of flower shots, I'd suggest Nikon's 60mm f2.8 AF-S micro. As Eric has mentioned, stay away from the filters and adapters. They are cheap, but not as good as a dedicated macro lens.

For portraits you could start out with a 50mm f1.8 or f1.4. On a crop sensored camera like the D90, it's a great general purpose portrait lens. No zoom needed for portraits.