Hello
New here myself, but from what I've seen this is a great forum to be on.
Depends what you're wanting to do. Are you collecting in the hope to make money from selling your pieces later? If so, I'd do something else as that's a bit risky when you are new to this.
What I hope is you want to collect because you see something and say "wow! I love that!" That's a good reason to collect.
Set a budget what you're happy to spend and then stick to that. Some people spend way to much and can't afford it.
The forums here are full of interesting threads about many pieces. So first off have a search for whatever model you're thinking of and see what other people think of it. If there is no thread, put up a post in the appropriate section and see what people think.
Depending on how you want to display you may want to settle on a scale. Most popular is 1/6, but lots of people seem to be upsizing to 1/4 scale or even bigger. Or some people mix their display with different sizes. Personally I like the bigger models as you can see more on them. But that's just me. Others will say 1/6 because there is a bigger range, they are easier to sell on and they need less display space.
So to buy new or old? Depends what you want. Focus on what characters or movies you really love and then see what is around for those. New has the advantage of a guarantee and that your piece might increase in value over time (might fall too though!), old is good for certain pieces no longer made. Old will often come from individuals, so check feedback on eBay or on here and buy carefully.
But stick to buying stuff that you love the look of. And if you fill up with stuff and want something else then consider selling something before adding more.
Hope that helps. And hope you have fun with your new and potentially hugely expensive hobby