It's a baaad idea (sorry), because for one thing, there's no drain holes and without any, the roots will end up rotting. For another, the air circulation is bad... and any sunlight will be enhanced - too much - by glass, and burn the tree or encourage mold inside. And, in any case, bonsai's all about trees in low, wide pots (literally 'trays' in Japanese), so they wouldn't really be bonsai in the end anyhow. But.. creative idea I guess.

Really? because its actually growing better then it was. leaves are sprouting everywhere. when it was in the pot it wasn't growing at all and i was having different problems (bugs and yellow brown spots on the leaves). but i do understand what you are saying. the glass would enhance the sunlight. i live in Indiana, dang near close to Chicago and this weather has been really trippy. Sometimes sunny, sometimes not.

i don't water it too much because i know of the problems with the drainage. i put rocks at the bottom also. i actually got freaked out when i seen the roots were white. i thought something was wrong with it. (used to seeing brown roots)

but i know i have to change it because its not really good for the tree.

another question i wanted to ask, what kind of fertilizer should i use?

The new growth is most probably a temporary effect of alleviating a previous problematic condition, and will soon return, and with much higher severely.

A good fertilizer is one that mimics the natural NPK ratio in most health plant tissue. One with a 3-1-2 NPK ratio (3-1-2, 6-2-4, 12-4-8, etc,) and contain micro nutrients as well, especially Calcium and Magnesium (in a 4-1 ratio)

I use Dyna-Gro "Foliage Pro" (9-3-6) and supplement occasionally with Dyna-Gro "Pro-TeKt" 0-0-3 to provide more potassium and silicon for stronger cell walls in the plant.
Both can probably be found at your local hydroponics shop or on-line.

What KIND of tree is it? That matters a lot to the future because if it's a tropical, it will do well for now in the jar, but not so good in future for various reasons. If it's anything else, and needs to be outside all year (certainly fall thru spring to experience going cold gradually, being dormant for months and then warming gradually in spring) it won't last long. You must know what you're growing... 'bonsai' is a technique, or collection of them, not a type of tree.