Ok, I got the bike, what do I really need now? I am planning on getting a helmet, now what?

Bike shorts and gloves are next IMO, also sunglasses (with interchangeable lenses for sun and low light).

While not as essential as a helmet, your body will thank you after a few miles if you get this stuff!

I vote a good pair of shorts (with the pads in it). Your rear will thank you later. :)

Then a camel back or any of the bladder systems. You can go further if you drink more water.

^^^^ Bike Shorts with the butt pad will make it so much better.

I would also get a Bike multi tool.
and a Camelbak.

Maybe some gloves while your at it.

Assuming you already have clipless pedals and shoes that will accept the cleats (no one should ride a trail without clipless), the essential items are:

Helmet

Eye protection

A Camel-Back or similar hydration system

A spare tube and air pump + plastic tire irons

Padded Shorts

Gloves

A Trail Map, unless youre familiar with the area or it is well marked

A cell phone (if riding alone)

I never used to carry a cell phone with me, and often ride alone during the week. I had a front tire roll off the rim on a steep downhill a few years ago, and separated my shoulder. I had to walk back 3 miles with a useless arm and a broken bike over the other. It really made me realize that I could have been out there a long time if I wasn't able to walk back.

camel back, cycling shoes, maybe clipless pedals, stuff to change a flat, lube for your chain, shorts, jersey, gloves, and band-aids and beer for after the ride. or during. we've been known to do that.
ski season is almost over. time to do all the things to my mt bike that I should have done at the end of last season.

Helmet - with name, contact number and bloodtype inside
Shoes with cleats and clipless pedals
Sunglasses withi removable lenses with Yellow, Clear or tinted depending on sky/weather.
Camel Bak
1 or 2 water bottles; I use mine to cool me off and for help finding holes in tubes
Spare tube in a ziplock bag with some babypowder to keep it from sticking.
Patch kit incase you get more than one flat; it happens.
Energy bars
Multi-tool that includes a chain tool
Chain repair kit
Pump or CO2 refill kit.
Zip-ties
Cellphone
Elbow and shin pads
Mountain bike shorts; Pearl Izumi and Zoic are some of my Favs.
Bar ends are a personal preference.
Trip Computer
I pack everything into my Camel Bak Mule.