Take it from someone who has been painting off and on every summer since I was about 9 years old. When it comes to painting, inside or out (interior or exterior), there are some "Tools of the Trade" that will make your job a WHOLE LOT easier.
I've done several interior jobs, I prefer exterior...probably because you can be a bit messier without consequence.
PLEASE, if you're going to paint that house of yours yourself, take my advice and get yourself some tools (or borrow if you can):
You absolutely CANNOT buy one of those cheap brushes at Wal-Mart. Buy yourself a couple of really good brushes, clean them well, and use them the rest of your life (if you're not painting several houses per year). On our crew, a Wooster is the envied brush. My dad hides his until he's ready to dip-in so no one "accidently" grabs it.
I recommend a couple of 3-4 inch flat brush (one for primer, one for paint) and a 1 1/2 - 2 inch angle brush (for trim).
This will be used at the end of each day to clean that $30 brush you just bought.
Ice Cream Buckets
Depending on how many colors and how many people will be painting, start saving and have several on hand to pour your paint into.
A fancy wire hook that goes on your bucket hook so you can hang the bucket off of your ladder. You can also use a wire handle from a left-over paint can.
The reason most people hire it done. You will need a good 5 rung step ladder and at least one size of aluminum extension ladder. For high peaks, you may need an additional size (I've seen some people rent boom trucks).
This is a must-have attachment for your extension ladder. It props the ladder off of the house and gives you stability. It allows you to prop on window frames for hard to reach areas.
Dont' spend a bunch of money on those plastic or canvas cloths at the Hardware Store. Raid your closet and donate an old sheet or blanket to the cause. Otherwise check-out your local thrift store and buy a cheap blanket or two.
Paint Can Opener
You can buy a little tool just for this job for about 44 cents, or just use a flat-head screwdriver, a butter knife, flat rock...desperate times call for desperate measures. Nothing like being ready to paint and not being able to open the can!
The very first tool you will need. Don't forget to scrape! I prefer one that does not have a moving head and has a scraping edge on both sides. In this case, bigger is not necessarily better. That thing will get heavy in your hand real quick!
To get paint off the windows, nearly impossible to avoid.
Rags & Water
Spills are unavoidable. Have a few rags handy next to a bottle of water so you can wet the rag. Act quickly, paint dries faster than you would think. If you spill on a sidewalk, flush it with as much water as you can quickly.
To wrap wet paintbrushes or buckets with paint in them while you're on break or using a different color. This will keep everything from drying out. Place them in the shade.
Spray Paint that matches shingle color
Ideally you will have drop cloths on your roof, but just in case you need to cover up a paint splatter, a light coat of spray paint does the job.
Sunscreen or other Sun Protection
Ideally you will work with the sun and stay in the shade as you make your way around the house, but if you have to be in the sun, it won't take long to develop a good burn. Sunglasses are a must if painting in the sun, the glare from the house will nearly blind you.
Wasp nests lurk in many places especially under eaves. You don't want to be caught up on a ladder dodging a stinger!
Again, on a ladder getting bit by flies, gnats, or mosquitos is not a good idea!
To keep paint chips out of your eyes. Many times you will be scraping above your head. Trust me, paint chips HURT, really BAD!
Not the religious type, a handkerchief or old baseball cap will do. Without it, you'll be digging paint chips out of your scalp for days and picking dry paint out like monkeys picking lice for quite some time.
A primer with stainblocker is best. And only enough to prime over bare wood after you've scraped.
Paint (of course)
I have a few favorite brands including Benjamin Moore (my parents preference especially for historical colors), Diamond Vogel, and most recently I've added Valspar (available at Lowe's. There really is a difference from a painter's stand-point. They are nice and thick, cover well, and brush on smooth without dripping everywhere. They are yummy. Evidently they last longer too.
Okay, not really a "tool" so-to-speak. But without them, you will wear out quickly. Don't try to set records. Take a break every couple of hours and stay hydrated.
Did I forget anything? I can't wait to hear how your house painting goes!