Hi and welcome
Ditto to all the above posts.
I'd recommend starting with an assault troops squad of ten. It's fast, scoring unit, can split into combat squads and not too bad on points cost.
Then perhaps a Rhino. I am a fan of being able to move around your troops / scoring units quickly.
From the Black Reach box set you get the dreadnoaught - that's a good fire power and assault unit - if you can get it to live long enough:)
Start with these basics and see where you go. Don't go crazy or broke adding more stuff just to get into the arms race. I enjoy the building and painting, so I usually add a lot of units, but for me, that's a big part of the hobby. For the gaming half, keep it basic until you find your way.

****
Warhammer -- too big to fail