Welcome back to Guard! Its great to see another mass infantry player

I have been running pure infantry guard for 5 years now and I can say with confidence that it is still going strong.
However, the
40k has changed in 6th and 7th edition. Assualt is much harder to do as
IG now and shooting is more viable. With that in mind (and the models you have) I suggest the list below:
Combined Arms Detachment (AKA the old style force org chart)
HQ
Company Command Squad 1: Lascannon team, Vox and Sniper.
Company Command Squad 2: Lascannon team, Vox and Sniper.
Priest: Autogun, laspistol and
CCW.
Priest: Autogun, laspistol and
CCW.
Priest: Autogun, laspistol and
CCW.
Commisar: Power Axe, Bolter.
Commisar: Power Axe, Bolter.
Commisar: Power Axe, Bolter.
2 command squads will give you plenty of orders to buff you men with and the Lascannons allow them to snipe tanks from far away. Priest and Commissars are needed to buff the platoons
Elites
Tempestus Scion Platoon:
Tempestus Scion Command Squad: 3x Melta Gun
Tempestus Scion Squad (AKA storm troopers): 2x Melta Gun
Tempestus Scion Squad (AKA storm troopers): 2x Melta Gun
Tempestus Scion Squad (AKA storm troopers): 2x Melta Gun
These guys are you suicide melta squads, they drop in and blow up heavy armour. Consider allying them in, they have there own codex as well as being in the Guard codex. IMPORTANT NOTE: They are expected to die, anything you get out of them after they have completed their drop is a bonus.
Troops
Platoon 1:
Platoon Command Squad: Vox, Flamer and Autocannon.
Squad 1: Flamer and Autocannon.
Squad 2: Flamer and Autocannon.
Squad 3: Flamer and Autocannon.
Platoon 2:
Platoon Command Squad: Vox, Flamer and Heavy Bolter.
Squad 1: Heavy Bolter and Flamer.
Squad 2: Heavy Bolter and Flamer.
Squad 3: Heavy Bolter and Flamer.
Platoon 3:
Platoon Command Squad: Vox, Flamer and Heavy Bolter.
Squad 1: Heavy Bolter and Flamer.
Squad 2: Heavy Bolter and Flamer.
Squad 3: Heavy Bolter and Flamer.
Lots of bodies with decent firepower, this section of your army is primarily focused towards anti infantry. They will die in large numbers but will stand to the last man if blobbed up and a priest and commissar is put into each blob.
Heavy Support
Wyvern
Wyvern
Hyrda
OR
Hyrda
Hydra
Wyvern.
It depends on your meta on which combination your go for. If you are facing lot of infantry i recommand option 1. If you are facing Flyers and Skimmers I recommend option 2
That is a 1500 point list using only the models you listed. It is not the best list out there but it covers most bases and allows you to go and start playing games without buying a single model