You'll probably want a high quality down bag like this one from Mountain equipment shop
A fiberfill bag will be warm enough except in late December and January Insulated jacket
It's cold up there; be sure you have a warm insulated jacket
If you're not going high, a lighter jacket will be enough. You won't need one in April and May

Fleece jacket
Fleece is light and stays warm even when wet.
Try a windproof version

Duffel Bag
If you are traveling with porters, you'll need a strong, waterproof duffel bag for your gear.

Daypack
If you have a porter you'll need only a daypack
If you're carrying all your gear, you'll need a larger pack

Extra equipment

Water Bottle

Don't forget this. Forget all the complicated stuff, buy a good Nalgene bottle that does not leak and you can take it to bed as a hot water bottle.

Flashlight or Torch

A small hand-carry flashlight is a good choice. Be sure to put a string on it so you don't drop it.
Though many people prefer a headlamp - which is easier to use when you are reading and bed (and you don't risk dropping it into the toilet!).

Walking shoes
High top shoes provide ankle support
But many people prefer lighter, low-top shoes.

Boots
Boots are a better choice if you are going to high altitudes
Here is the women's version

Gore Tex Parka
It's expensive, but Gore Tex keeps you dry Women's Jacket

Less expensive rainwear

Or use a poncho, which is cooler in hot weather
Sunglasses or goggles
Get a pair with side shields like these
Shorts, Shirts and hats

Socks
Pocket Knife
Preferably with scissors
Sandals for Camp wear

Walking Stick
A single walking stick
Or a pair of poles
Navigation Aids
Altimeter
Or an inexpensive GPS

Other equipments :-

Clothing
Comfortable trekking clothes, sufficient underwear to wash and change, raincoat, umbrella.

For the Night
warm night dress, torch, toilet-paper, thermo matrices, head pillow, oropax.

Safety / Health
Money belt, spare money, credit card, copies of important documents, medicine box, crèmes and tapes, antibiotics, antihistamines and antidiarrhoea. sun protection, hat, lip fat stick, electrolyte minerals.

Presents
Writing and painting material, candies, torches, crèmes, tobaccos, Polaroid photos, medicines, second hand clothes and shoes.

Others
Walkman or disc man, batteries, books, enough money in small amounts, Go for the Trekking Information.