Several things here.
First off, a lot depends on your locataion and that of the radio stations you want to receive. Use this little toy to see what's in your area
hint: Use the "advanced search" option and search on your zip code.
http://www.radio-locator.com/cgi-bin/finder?page=Y
This should have given you a listing of stations, their location and the distance between you and them. you know the local terrain better than I so I'll just talk in generalities.
Flat terrain or water between you and them is the best. If there are any hills, mountians, large buildings and such between you and the station, your reception will be hindered.
Likewise, distance plays a part here. The farther you are, the more antenna you'll need.
If you want a cute little thing that sits on top of your set, fine, but don't expect good performance. No tiny piece of metal with an amplifier is any match for a "real" antenna, which I'll discuss below.
If all the stations are in the same direction, you can use a directional (yagi design) antenna. Like a magnifying glass, it picks up stations at a greater distance, but in only one direction. It basically ignores stations to the sides or back. These come in all sizes. The bigger the antenna, the greater distance they will serve.
If you live in an urban or suburban location with stations all around you, you may get by with an onmidirectional antenna. It doesn't have the range of a yagi but it picks up equally well in all directions. I'm right outside of NYC ane these serve my needs nicely.
Here's an example of each :
http://antennacraft.net/FM.html
Both of these should fit in an attic, but the higher they are, the better they will perform. And I hope you don't have much metal in your building or the sides. That hurts their performance.
So much for FM. AM Antennas are a different matter entirely. These are a single wire, fairly long, and strung between two points. Maybe a run in the attic might help, but it should be perpindicular to the stations you want to receive. Ohm, they require a separate feed, generally one unshielded wire. I'll leave further discusson of these to more knowledgeble persons than I.