Welcome! There are many general NESing templates that we use.
Broadly speaking, the simplest kind of template might look like this:
Nation Name: Nessos
Economy: Large (6)
Military: 12 000 Infantry, 2000 Cavalry
Economies will often be modeled by a number of points or levels. They can be spent on various things, and military stats broadly show how much force a nation can use to defend, attack, or intervene in the affairs of other countries.
Of course, a stat block that simple wouldn't model much more than a simple war game. A conventional NES might have a variety of lines to describe some cultural details and provide some information to flesh out the country. Here's a system that I've used in the past:
Nation Name
Cities: Capital, if any yet. Major cities will also be listed here.
Leader: Your Nation's Leader/Your Name
Culture: A sentence or two describing your nation's ethnic and religious makeup.
Government: A sentence or two describing your nation's political structures.
Economy: 2- 2/0 [Note: this indicates two economy points that can be spent, and a breakdown of how much of the nation's is based in rural areas, and how much is based in urban areas]
Population: To be determined by me
Army: To be determined by me
Navy: To be determined by me
Technology: A sentence or two describing your nation's technological and scientific development.
Wonders:
Description: Please describe your people's appearance and any interesting things about them.
An example of this is shown below:
Annokha
Cities: Kettedin
Leader: Assorted Chiefs/North King
Culture: Annokha Animists, with large enslaved minorities.
Government: Originally a consensus-based tribal government, war chiefs have found ways to subvert the authority of the political leaders and have in effect set up a parallel, more powerful system of government.
Economy: 2- 2/0
Population: Small
Army: 1500 (Horsemen and Archers)
Navy: None
Technology: The military is fond of horse and short bow. Domestic technologies are fairly simple. Pottery and agriculture are around, of course, but the margins of society still hunt and gather to supplement this. Agriculture still uses the foot plow, construction is only at a basic level, and they generally have to trade for metal tools.
Wonders: None
Description: Annokha are an unsympathetic, fierce tribe of slave raiders from the hills they call the Tambuk, green and cool compared to the lowlands nearby. Expansionist and ruthless, they are in a state of mild hostility with most of the people they meet (they don't really regard raiding as synonymous with 'war'; war is an altogether different and much more destructive endeavor), but simultaneously quite eager to cultivate trade links. However, their capital at Kettedin is a place where any merchant can find a safe haven, and in general, men with whom they have a good rapport will almost certainly not fall victim to their wrath.
This system allows for the clear naming of major cities and a brief description of the culture of the nation, an abstraction of a people's economy and military capabilities, and a more detailed description of the nation as a whole.
Here are some other systems, this is North King's from N3S III: End of Empires:
Haifaio Hailsia - Lord_Iggy
Leader: (Age: )
Government: Gerontocratic Oligarchy
Factions:
Aramsayafa Faeoria: Ruling Faeoria. Large, somewhat chaotic.(Leader: (Age: ))
Faerouhaiaouans: Culturally influential but recently politically marginalized; disorganized.(Leader: (Age: ))
Dremaiites: Rich, influential, but somewhat disenchanted.(Leader: (Age: ))
Ieraitans: Extremely influential intellectual/philosophical movement based in Sakhelakheia.(Leader: (Age: ))
Culture Description: Hailsia is, quite possibly, the center of the world. Faronun theater is performed the world over, and its art and architecture has imitators as far away as Parthe or the Trahana. The counterpoint of Kothari music has begun to have a significant impact on what was once a fairly conservative style. Doru o Ierai, or the Hunt for Truth, is the focus of many in the country, who seek to spread the "perfect knowledge" through the world, especially to the south. Recent cultural exchange with the Vellari Exatai has caused a cultural flowering, particularly in theater.
State Religion: Conclave Iralliam (Doru o Ierai)
Annual Income: 44165
Annual Expenses: 30000
Expenses:
Treasury: 10750
Debt: 0 (Interest: 0 (Rate for new loans: 0.14))
Military:
Army: 50000 Infantry, 10000 Cavalry, 0 Levies, 0 Siege Train(s)
Navy: 200 Ships
Maintenance: 30000
Description: The Haifaio Hailsia is a large empire centered around the peninsula of
Hailsia, regarded as the successor to the more ancient
Empire of Helsia (though they claim a continuity of government), and a breakaway state from the former
Farubaida o Caroha. Established after a long period where the Farubaida drifted increasingly apart due to differing geopolitical concerns and visions, Hailsia championed the growing spiritual movement of
Iralliamite schismatic movement, the
Faerouhaiaou and the
Empire of Dremai. Today, the Haifaio is a vibrant cultural center, especially around the upland city of Sakhelakhaia, and a significant trading power.
This system shows a detailed breakdown of internal factions within a nation and has a much more granular economic system, with maintenance expenses for keeping large armies and navies, but fundamentally shows the same basic things.
And here is EQandcivfanatic's system from Capto Iugulum:
Vinland: Lord_Iggy
Formal Name: Kingdom of Vinland
Parliamentary Monarchy: King Ingvar Gyllensvärd (Conservative)
Riksdag: Konservativ Industripartiet (Conservative)
Head of Government: Statsminister Konrad Franzen
Capital: Nya Stockholm
Manpower: 1,350 (+18)
Economic Points: 0 (+225)
Domestic Spending : -52 EP
-Education: -20 (14)
-Infrastructure: -6
-National Insurance: -14 (14)
-Vinlandskridningpolis: -12
Royal Vinlandic Armed Forces: 104 Infantry Brigades, 42 Cavalry Brigades, 11 Lodjur Armored Brigades, 6 Lejon-Modell Armored Brigades, 1 Björn Armored Brigade, 4 Rovdjur-Modell Armored Brigades, 19 Artillery Brigades, 2 Landcruisers
Army Quality: 8
Army Doctrine: Kungliga Vinlandska Försvarsmakten 1930
Royal Vinlandic Navy: 1 Trovão-class Destroyer,7 Furacão-class Destroyers, 6 Tufão-class Destroyers,1
Maryland-class Battleship, 1
Kronos-class Battleship
Navy Quality: 5
Royal Vinlandic Air Force: 1 P-16 Fighter Squadron, 1 P-21 Fighter Squadron, 5 F-25 Fighter Squadrons, 4 JC-7 Fighter Squadrons
Air Force Quality: 2
Once again, the system shows a brief explanation of the government, while also showing the available manpower, an abstraction of the effective 'size' of an economy, plus a fairly detailed military breakdown including doctrines and equipment models, befitting the alternate-1900s of the setting.
The rules that you use will vary heavily depending on what sort of game you want to run. If you want to run a 'fresh start' approximating a game of civilization, you might want something that has a focus on culture, social organization and what constitute's a given group's economy, while a modern wargame might be much more focused on very specific technologies, doctrines, command structures and force locations. What are you hoping to run?