Some tips on that:
If you have plastic glue (I dont really know what its called) You can stick that in a tiny jar and put sprue bits in it. It'll melt and turn into a sorta plastic putty that you can fix with, sculpt with , or sand down!
The site dalong.net has a ton of pictures of built old and new kits. You can photoshop them together to get an idea of what youre going to do before you kitbash.
ALWAYS save your extra pieces for later.
You may have heard a rumor that combining super glue and baby powder makes it into a putty you can sculpt with. IT. DOES. NOT. WORK!!!! (Yes this is speaking from experience...)
You really dont have to spend a ton of money on supplies, I use a cutter I got for free and a cheap 10 dollar pack of acrylic paint from michaels that works perfectly fine!
Make sure to find a hobby store near you, it saves a lot of time and money on shipping and its just cool to go there! I go to Gundam Planet.
EXPERIMENT A LOT. I'd say about 90% of my bought gunpla is gone or different, not cause im irresponsible, but I use it for other things or customize it!
Try using random things around your house for detail, legos work really well.