The boxed set comes with a typical set of ingredients for a space marine squad -- at least it would if it were ten millennia in the future. Each ten man squad gets some special weapons (melta, plasma, missile launcher, heavy bolter, flamer, combi-weapons) as well as some upgrades for the squads (power fists, lightning claw, power sword). There is also the terminator unit, the contemptor, the terminator captain, and the chaplain to consider as well.
To be honest, there's not much to play with here in a number of ways. We do not have enough special weapons to form a support squad out of, hence for the basic marines, it is going to have to be tactical squads, or veteran tactical squads (assuming we're not counting them as something special from a specific legion).
The other major negative is the lack of speed and deep striking ability to close the gap to the enemy. Hence this would be the first thing to invest in once you're looking at expanding the initial army. Certainly things like rhinos and other support tanks would be a sound idea.
Rite of War.
There is not much choice for the rite of war either. We either want an army without one at all (a distinct possibility), Pride of the Legion (so we can get terminators and veteran tactical squads for troops), or Chosen Duty (for a Delegatus HQ choice). With a bit of expansion (additional purchases!), Orbital Assault becomes a good option here to focus on and one that I can certainly recommend. But it will require something like drop pods to be purchased.
In the list below, I try to not incorporate too much conversion work as I want it to be suitable for a beginner. For the Rite of War, I'm going to select Pride of the Legion as the points cost is over 1000 points in total and therefore there's no real need to field a Delegatus.
This praetor provides the rite of war and should be fielded with the terminator squad below. The master-crafted status on the chain-fist is just for fun really and can be removed if desired.
Legion Chaplain, artificer armour, refractor shield, plasma pistol (120 points).
Field with one of the aggressive close combat squads below (or even the terminator squad).
Terminator squad, 2 chain fists, 3 power fists, 3 x combi-plasma guns, sergeant with grenade harness (241 points).
This is a slightly mixed purpose squad to field with the praetor. The combi-plasma guns come from the tactical squads and will therefore need some minor conversion work. The set up here is to be able to tackle enemy elites, as well as pose a strong threat to vehicles in close combat. In a pinch, this could be swapped to a command squad if desired but there's nothing to build a command standard bearer out of, without some moderate conversion work or kit bashing. So we'll leave them as a terminator squad. Naturally, the plasma components could be swapped out for melta guns for a different type of threat against tanks, but given how much armoured ceramite is around, I prefer the plasma option here.
Veteran Tactical Squad with 10 members, 2 with heavy bolters plus suspensors, 1 legion vexilla, sergeant with artificer armour and power fist (275 points).
Give this squad the sniper universal rule and take aim.
Veteran Tactical Squad with 10 members, 3 with power swords, 2 with plasma guns, sergeant with power fist, plasma pistol and artificer armour (300 points).
Go with fearless here to save points on the vexilla and team up with the chaplain. Get in to combat as soon as possible. The plasma guns can be swapped out with the melta guns if desired. Consider taking squad with melta bombs.
Veteran Tactical Squad with 10 members, 1 legion vexilla, 1 nuncio vox, 2 with missile launchers and suspensors plus flakk missiles, sergeant with artificer armour and power fist (305 points).
Take tank hunters here and stand back and shoot. Consider adding squad wide melta bombs here. In fact, I recommend taking the melta bombs on either this one, or the close combat squad above since you have some on the sprues.
Contemptor Dreadnought, Kheres Assault Cannon, Power Fist with heavy flamer (200 points).
I like this set up due to its flexibility. It can support every element of this army very nicely.
Total army points cost = 1613 points.
This army is slow and can be outmanoeuvred. It needs to be expanded with transports (rhinos, drop pods, storm eagles, land raiders) as well as other support elements (vindicators, predators, heavy support squads, Sicaran tanks, etc.).
In principle, one of these veteran squads could be switched out for a seeker squad which might provide an interesting option if given a rhino (etc.) to improve its movement.
For a legion specific option, the Night Lords don't look too bad if some of the squads are treated as terror squads. Iron Hands might be good for taking incoming fire despite being slow. Iron Warriors too to ignore some of the incoming fire's effects. Alpha Legion could be excellent here for infiltration purposes.
Drop some points if needed to make it under 1500 points by eliminating the chaplain. Expand to 1750 points by upgrading some of these squads (one with melta bombs everywhere, and perhaps another as a full seeker squad, or even a veteran squad where everyone has power swords?).