nothing blatantly wrong but do consider that the easy AI is very slow at colonizing and attacking (even with aggresive mind set). Therefore dont focus too much on defense and go offense. Even with a fairly good size system (20 to 25 planets) you can knock out an easy, normal, even hard in less then 2 hours. Insane can take a little bit more but even there only focus on forifying certain key planets (aka choke points) and you should be fine.

Against easy and normal AI it is usually sufficient to start building defense when the enemy arrives. He comes with a small fleet at the beginning and then upgrading Emergency facilities and building turrets, hangar defense and a repair platform at the planet under siege will be enough to fight him back. And in most cases it is not your HW the AI will attack first because there are colonies closer to him thus defending your HW is not necessary.

Therefore focus on economy and expansion, grab planets (first of all the asteroid next to your HW if there is one), build broadcast centers and trade ports and then crush him with your superior income by building massive fleets.

But to answer the initial question:

vs easy and normal AI I took all planets I could get before building defense  

vs hard AI I usually take (beside my HW) 2 to 4 (depending on the map size) planets before preparing defense.

I'm repeating a bit of what was in the previous posts, but here's a rundown for ya:

1. Get the capital (Colonizer Cap, it's the one in the middle, though you can argue for the Marza Dreadnaught (TEC Seige Cap at far right) but then you have to get colonizing frigates)

2. Scouts for scouting

3. A few light frigs, don't invest too much in these things, they aren't good

4. Look for your asteroid. In almost every game there is one asteroid next to your homeworld. Colonize it first. Easiest to take (only two pirates protecting it).

5. Do as you damn well please

Few tips though:

1. Don't over invest in stationary defenses. They don't do you any good early. In most games you and your ally are pretty far apart so your goal is to get a fleet going so you can expand quicker and get more planets. Save these for planets that your enemy is actually attacking. If you know you're not going to expand any further in a certain direction it makes sense to put up a few defenses (say a hangar bay, a repair bay, and a few turrets) so that your defenses can fight off the occassional seige frigate raid (AI loves to do that...annoying). My point? Put up defenses on planets that you have either been attacked on or know you will be attacked. Thinking of buying a fleet as buying mobile defenses.
2. Use LRFS when possible. These are the early game tanks. They are not a balanced class (unless ur playing one of the balance mods out there), and their strength is useful.
3. You can use light frigs if you'd like. I just avoid them. LRFS are so much better. Eventually you should make the switch.

Newbie here with a couple added questions. 

So I should not build defense platforms right away? I should invest in a capital ship and light frigates?

How about research? Should I try to get high research or focus solely on a big army in the beggining?

I have found that I can colonize a few planets relatively quickly, but I'm always fighting the pirates (even if I put bounty on my enemy) and inevitably my enemy shows up at one of my planets with a GIANT army. 

How many civic and military research stations should I build at the very early stages of the game?

thank you for your help

By the way, even though I'm getting my butt kicked I'm still enjoying this game. 

Personally, I answer what everyone else has posted with 3 things.

1. I usually get a Marza or Kol as my first cap, then buy a colonizer frigate. Because I need their firepower when blowing my way through militias as I aggressively expand in the beginning of the game.

2. I don't built defenses on my homeworld. I colonize outward until I find a planet with a decent choke point prospect and fortify it.

3. Starbases are better than a defensive fleet. The best strategy I've encountered with those is a starbase with fully upgraded weapons, armor, and as many bomber fleets as it can hold after that + fighter/bomber squads from hangars and minefields flanking the starbase. Also, in the spots flanking the minefields, defensive turrets are needed (but they're not of much use without at least the burst missile upgrade) to make the enemy want to go into an area "undefended" between the starbase and turrets... which is full of mines in phase space.

Anyways, point is, if you do it right, you don't need a defensive fleet. Unless you don't have Entrenchment. In that case, the words "the best defense is a good offense" are gold when it comes to Sins.