Car Buying Advice needed

i am wanting to trade in my car before it gets too many miles on it to sell for a good price, a relative that runs a dealership told me that it's worth $10,000 right now, and i shouldn't need to finance it. I have never bought a car before so im not sure exactly what the process looks like. could i get some advice? I really like Mazdas and I found one that is under my budget and brand new. What are good things to look into or ask about? what should i avoid?