I always recommend to first get acquainted with the lore and various army backgrounds. Space marines are a solid choice for a starter as they are pretty easy to learn with. Always go vanilla, ie ultramarines, imperial fists, salamanders etc. They are easiest to start with and you can branch off with them at a later date. When you learn a bit more and have more games under your belt, you might find yourself leaning more towards what you REALLY want to collect, be it orks, tyranids, guard, eldar etc, and by that time you should have enough experience to do it properly. I suggest to buy one of the starter sets and just start playing. The models there are more than enough to learn and have fun with, without being too expensive to sell off later or just put it aside to get your true army.