I think many trainees ( not only beginners but also many many advanced bodybuilders) do not buy the right items/products for themselves ( so it's normal that you have this question).
Many people tend to fall into the traps of buying what they don't really need. Like what is new in the market or what is been advertised in the magazines. For example, I saw alot of bodybuilders rushed for Nitrix Oxide (NO) products when this industry first lunch this item. Thinking that this product can produce wonderful result. I am NOT saying NO products are no good, BUT you got to know the basics like having enough protein, carbs, vitamin & minerals etc first, before having the more advance items.
There is no difference in between beginners, intermediates or advanced bodybuilders when it comes to "nutrition or supplementation" in building bigger muscles. But training does.
Even if you are a beginner, you can have all the supplement from basic to advance, if your budget allows.
Well !
What to buy?
-It depends on your daily meal plan or another word "your diet". Knowing what you eat daily - to determine if you are getting enough "healthy" protein, carbs and fats etc.
-It also depends on your body type, your body fat level,and also your activity level.
My answer is try the basics like whey protein ( scivation whey/ ultra whey pro ), xtend ( branched chain amino acids) or gluamine and multi vitamin-mineral (daily formula). They are basic products but very important.
It won't be wrong when you choose these products ( if you don't know what suit you ).