As soon as night temperatures in your area stay above 55 degrees start taking your tree outside and get it adjusted to the outside daylight. Start slowly by taking it out for a couple of hours each day increasing the amount of time it is outside each day. This is usually a two week long process or so before it can spend the entire day and night outside for the rest of the growing season.

When the tree first begins to get direct sunlight it might react a bit funny by loosing leaves. This is to be expected since the leaves on the tree that grow indoors cannot handle direct sunlight. Remember that as the tree is in direct sunlight is will begin using alot more water as processes in the tree speed up. Keep an eye on it more closely because depending on the soil your tree is in you tree can dry out in a single day and suffer damage or die.

I would begin fertilizing as soon as there is two or three full sized new leaves for each branch where the old one may or may not have fallen off from. Since you are just getting started and are learning your trees requirements I would start with and organic based fertilizer like Biogold which is a complete fertilizer that contains all micro and macro nutirents. It's hard to ruin a tree with this type of fertilizer.

The key to getting your tree to thinkening up is not to trim it. If your tree has the basic shape you are already pleased with and don't want to loose it you will need to trim it after it has had a growth spurt but deciding how much and when to trim compared with how much you want it to grow is the tricky part you will have to figure out as you grow and your tree grow.

One more thing about ficus' you may want to keep in mind is that the root zone will also be growing. As it does you will need to keep an eye on the watering because ficus' can and do sometimes become root bound over a single season of growing especially since your is a new one it may be difficult for you to know just how much roots it has right now. So if when your watering during this time you are trying to grow you tree out a bit more and you notice that the water isn't draining freely through the soil is may be that it is becoming root bound. I suppose you'll cross that bridge when you get there though.

Have you been able to get any bonsai books yet? -G