Start out with a centurion list. (ca 1500p only infantry, dreadnoughts and speeders allowed).
I suggest to build and paint up the following as a starter, play it a few times and then get a feel for what you want to build and paint after:
A warmonger (the legion centurion)
10x plasma vets or plasma support squad (magnetise the wrists and weapons with 2 mm diameter N52 strength magnets).
5x Gal vobak, meltagun and sgt with powerfist
Overall game strategy suggestion:
- Your tacticals focus on the objectives.
- Your warmonger deep strikes in within rapid fire range with plasma vets and reduces the most threatening enemy unit.
- Your Gal vorbak deep strikes in and fights dreadnoughts. (the squad has a S10 powerfist and a meltagun)
This is a quite strong list, I suggest you magnetise the plasmaguns/combiplasma on the veteran squad to adjust the power of the list (see youtube for magnetising tutorials). At 1500p you can add in some more units, perhaps javelins, or assault troops.
The 30k community is quite fluff, modeling and painting oriented. You will enjoy the most respect and goodwill if you put in the hours and effort to paint your army well. So focus on getting the army built and painted first and then play. You will always need 20 tacticas for playing the objectives and a HQ, so start with those units. Leave the GAl vorbaks for last since they are moredemanding to paint (you should learn to paint skin nicely before painting the Gal Vorbaks).
There are tonnes of good horus heresy word bearers painting tutorials on youtube you can watch for inspiration.
I also strongly suggest to throw yourself into airbrushing and airbrush the army. Space marine armies are as if custom made for airbrushing. Get the two GW word bearers red (one is darker for shades) and also look into weathering such as chipping, oil paint pin washing etc.
Good luck and welcome!