My character uses forsworn armor, but I don't want to use any helmet. However I like the way circlets look on her. Can I enchant these?

She is a forsworn armor wearing, warhammer wielding character, any ideas on what I should enchant it with (if possible too)

You definitely can enchant circlets, with the same things you can put on any headgear as far as I can tell. I can't remember what enchants go on what bits of armor (it varies) so I can't really recommend, sorry.

Any ideas on what enchantments I should use on any pieces of armor?

Forsworn is light armor, right? You can find enchants to improve your light armor rating and two-handed damage. I'd also recommend some magic resistance (this goes for pretty much any build). More health also, perhaps? I dunno, I'm not a min-maxer so you might get some better answers from someone who is.

for enchants for a Two Handed Warrior i would suggest some of these Fortify Armor, Health, Block, Two-Handed or Resist Fire, Frost, Shock, Poison, or Magick. Now which ones you use are entirely up to you and i can't really suggest which ones exactly to use since i am not and never will be a warrior.

I rocked the forsworn for quite some time (then went back to upgraded Saviour's Hide).
Circlets can only take Magicka-based enchantments like fortify Magicka and reduced spell costs (as well as some miscellany like waterbreathing). Not necessarily the best choice for a warrior, aside from aesthetics.
You can enchant the armor with a bonus to Light Armor skill and get it pretty effective even at mid/high levels.
I would recommend a Fortify two-handed on your gauntlets and some sort of elemental resist on your boots.
Not having a shield is rough at times, in particular, if you want a resist magic enchant, it has to go on jewelry.
http://www.uesp.net/wiki/Skyrim:Enchanting_Effects