From a fellow rookie who's blundered enough times and had some good advice from this board, here's some stuff that works good for me:
Paint your body. Even if it is already white. Get some white "primer" (actual primer) spray a thin coat that adequately covers, with an even white tone all around, and then take your gloss white spray can (lacquer is easy and dries quick), hold the can about 1' away and spray fast, quick bursts on the side, other side, top, front, back. Wait 5, then repeat.
Many thin coats! Your white will come out more even and prevent runs.
Easy as that. Practice on a scrap piece of whatever to get the hang of it.
For your tree parts try whatever you find easiest. Me, I build an assembly first (ie engine block, rear axle, etc.), spray it a base color and then detail paint on top of that. Others paint each individual part on or off the tree before assembly. Don't forget to sand those plastic nubs off when you remove the parts (with clippers, scissors, knife). Something I overlook time to time (and it makes for a sloppy job!)
When sticking this thing together, I just use that cheap testors tube cement for plastic-plastic. Just scrape the paint off your joint surfaces first. CA glue for parts that need fast stick'um. Use in small amounts. too much globs and ruins a tidy job (believe me, I know!) Use clear acrylic paint, white school glue or "window cement" for your windows. Other glues will haze and yellow where they touch.
Don't give up. It took me several tries on my first model to get it right (and even then it didn't look that great). The second model was more encouraging. Learning from the mistakes and all.
Good luck.