How to start a new army

Hey, I’m looking to start a new 40k army but I’m not really sure where to begin.

Should I start with a Combat Patrol or just buy units I think look cool? How many points should I aim for at first (500? 1000?) And is it better to build around a detachment/playstyle from the start or figure that out later?

Also, any tips on avoiding wasting money on stuff I won’t use would be appreciated (I waisted like 100 quid on stuff for ec that I dont use)

also what Second army shud inget i currently have 500pts emporors children and a lil bit of dark angels

Thanks all!