Help with the basis of a list

I've been into the painting/collecting/lore side of the hobby for years and have decided that this year I'm gonna push myself into actually learning to play (I'm super nervous). I understand the basics to list building, but I'm stuck at a brick wall.

I've got a flesh tearers army but I'm kinda lost on what to do when it comes to making a list. My plan is to run them as blood angels and include some blood angels units in these lists and other times just run them as ordinary marines. Is this a problem in any way? This is the best way i can explain Thanks