for the sand paper, you wanna kind of work through the grit. which means you start with a lower number(on the back of sandpaper there is a number)and then you work to a higher number. look at a few different types and give them a feel with your hand, you want something that can take off the extra material without gouging too deep. after you take off some stuff you get a higher grit(number)and smooth everything out, next is the finisher which is usually a pretty high number and you cant feel too much grit on it. this will make it really smooth.
for the paint i usually paint with model acrylics from a brand callled Tamiya. OR i just get really cheap acrylic paint from an art store like Michaels(usually around $1.88 a piece in the U.S.) The trick to painting is to make sure you spray a base coat(its usually a grey color) and make sure arent going too thick and letting layers dry out in between coats. you can use water to thin out most acrylics, you want it be about the consistency of milk or even just a bit thicker. if you dont let it dry properly when you go back to paint another layer you usually end up pulling up the last layer and getting it a little clumpy. Acrylics dry pretty quickly, roughly around 15-20 mins in 75%Fahrenheit. i usually put on a show in the background, paint a little, watch the show for awhile, and then remember im painting and go back to it.
I dont like to use enamel model paint, because on some plastics it never feels like it drys out and just stays sticky/tacky.
you can also use spray paints, but i usually prefer to hand paint items. just make sure its a product that is good to use on plastics if you are using plastic, metal for metal, etc.
Also, when you are done with the painting(even if you hand paint it) if it doesnt look shiny or glossy enough they have spray paints that are clear glossy. whenever you spray anything i recommend being about 5-8 inches back, and have a test subject to spray on first to make sure your can isnt clogged or spraying out clumps.
hope that helps