I have a friend that claims GMing is the best way to learn.
1. I thought up a level 10 Catfolk Sorcerer(female) as Npc,
but don't know enough to not go overboard.
2. I want to do a character as Kitsune rogue- to eventually
become an Arcane Trickster. I need advice on the best way
to build and work up to that goal. The YouTube video was too confusing
The best place to start is with the Guide to the Class Guides that is stickied at the top of this page. Some folks might be willing to build something from the ground up for you but most prefer that you have at least the framework for a character before offering build advice. The guides are excellent resources.
1) The key with Sorcerers is spell selection. This will be a little bit tough with the Catfolk, as characters with human ancestry can learn an additional spell every time they level-up as their favored class bonus, but it should still be doable. You want to focus on versatility and flexibility, picking spells that complement each other. If you want to use damage-dealing spells, ensure you take the Blood Havoc bloodline mutation at 1st level and the Spell Focus (Evocation) feat; without bonuses from bloodlines and feats, damage-dealing spells are very weak and not worth spending your limited spells known on as a Sorcerer.
Other than that, it really depends on what kind of preferences you have. Sorcerers can't do everything, but they can be good at anything.
2) The general approach with Arcane Trickster is to take 4 levels of Wizard (or another spellcasting class of your choice) then take one level of Rogue and take the Accomplished Sneak Attacker feat. This lets you meet the prerequisites and get into the prestige class. If you want to take Rogue at 1st level that's doable, but you shouldn't take more than one level if you're aiming for Arcane Trickster. The Arcane Trickster is a spellcaster first and foremost, and multiclassing in non-caster classes should be minimized as a result. If you want to focus more on the rogue side, I recommend either using the Bard or the Unchained Rogue with the Eldritch Scoundrel archetype.
If you want to be a Wizard, there is the Keen Kitsune alternate racial feature that lets you switch your charisma bonus for an intelligence bonus.