素人 is a person who is not skilled or proficient. Someone who has no expertise in a certain profession. =an amateur. E.g. 素人の書いたような文章。(=A piece of writing that looks like written by an amateur.) E.g. 素人には分からない微妙(びみょう)な違い。(=A subtle difference that amateurs cannot tell.) 初心者 is a beginner, a person who has just started learning something (or someone that's at that level.) E.g. 彼は運転（うんてん）初心者だ。(＝He is a driver with little experience.) I hope this helps :)