that rl-19 and h4831 are very good powders to start with they will cover from 308 up
to your 300 mag and not give up anything to other powders

the best wy to figure all this out is to read and ask questions
and you have found the best place to do that.

pick one component and figure it out [i would suggest cases,how long, etc]
and move to another component as you get comfortable with what you are doing
save bullits/boolits for last these are the most fun

runfiverun