Well, I’m very new to Forza and Xbox. 
And I need some Tuning/Upgrading tips, I also need tips for good cars. Once I beat the game i’m probably gonna buy a Bugatti, But i’m wondering if I should buy something else And also, Should I download my Upgrade’s/Tune’s or make them myself? Thanks 
Unfortunately, there are no simple, bite-size answers to these questions, except one: Don’t buy the Bugatti ;).
All are worth asking though!
I’d spend some time reading the forums, and check the subforums for tips on tuning and painting. If you post more specific queries, plenty of folks here can be very helpful with tuning and driving tips.
As you said, you’re new to Forza, so my best advice is just get out there and drive. Many of your questions will answer themselves!
Some cars I recommend:
D class: 77 Firebird, '88 VW Scirocco 16V (DLC), Renault Alpine (barn find)
C Class: '69 Charger, '88 Monte Carlo (DLC), Alfa Romeo Quadrifoglio Verde
B: Ford Escort Cosworth, Nissan R32 or R35, Chevy Camaro
A: Lamborghini Countach, Corvette C7, Audi TT
S1: McLaren F1, Ferarri F50, Lotus Esprit V8 (DLC)
S2: Lamborghini Veneno, Ultima GTR
I like these cars as they don’t require much tuning (if any) to drive fast, in the classes I’ve listed them in. Just upgrade sensibly: handling, acceleration, power, in that order. Ignore my sig, I just like the quote lol (I’m serious about the bit in caps though).
Enjoy
Thanks so much! This helped alot 