[Numenorean]

Hi Tuor, good to see you've started this ambitious and facinating thread.
The First Battle, as Rumil has already pointed out...

shows that the orcs, even at this very early date, were reasonably well equipped with armour and weaponry forged at Angband

Agreed, they were also revealed to be great in number and disturbingly well motivated for War, Silm.:

Now the Orcs that multiplied in the darkness of the earth grew strong and fell, and their dark lord filled them with a lust of ruin and death

...a great army came into Beleriand and assailed King Thingol

Orcs, who were shod with iron and iron-shielded and bore great spears with broad blades

We can discern that they posed a genuine threat not just to Thingols realm, but to all Beleriand, so just how were the vastly outnumbered Sindar able to realistically engage such well prepared foes? Well before Morgoth had even returned to Middle Earth, on the basis of prescient Dwarfen warnings about 'evils of the North' that had not been rooted out, the Silm states:

Thingol took thought for arms...and these at first the Naugrim smithied for him

therefore

Thingol's armouries were stored with axes and with spears and swords, and tall helms, and long coats of bright mail

As Tuor said earlier in the thread, Thingol is shown in an active light by his actions and preparedness. The Dwarves are also invaluably active at this time, they provide the Elvenking not only with excellently wrought weaponary, but also with intelligence on enemy movements and an elite auxilliary force to contain any Orcish retreat or Eastern expansionism.
Tuor

Was Morgoth also militarily an amateur here? Why would the orcish forces have been taken by surprise by Thingols forces?

Morgoth may have been testing the strength of Beleriand and found it to be more than he expected, particuarly that of Naugrim-armed Doriath. I would also assume that the Orcs were taken by surprise for two reasons. Firstly, the forces of Denethor and Thingol (which must have included his mighty Captains Beleg and Mablung) knew the terrain infinately better than the Orcs did, thus allowing for swift and concealed deployment at an optimum location for ambush. Secondly, the Elves would most likely have engaged the enemy with arrows in the first instance - the onset of which would deliver sudden shock to the enemy and maybe result in the errosion of Orcish command and control.
As we well know, primarily due to their lack of sophisticated military equipment Denethors host was all but destroyed, however the sheer size of the Orcish army must also have been a crucial contributary factor in his defeat as the Silm. declares:

Thingol came upon the rear of the Orcs and slew them in heaps

In this grim statement, 'heaps' is a visually evocative word and it offers a dark insight into Denethors last stand on Amon Ereb. I envisage that after the first few salvos of arrows were loosed, the iron shod Orcs simply overwhelmed his light-armed Elves of Ossiriand through force of numbers.
However, in the aftermath of the Eastern battle - and utterly contrary to Morgoths designs, Doriath was in fact stronger and far better guarded from anything overtly military that Angband could now throw at it, for Thingol bolstered his forces by accepting many of the Ossiriand Elves into his realm, yet more importantly still:

Melian put forth her power and fenced all that dominion round about with an unseen wall of shadow and bewilderment: the Girdle of Melian, that none thereafter could pass against her will or the will of King Thingol