the pot it's in looks extremely small and is likely constricting its growth. I would move it to a larger pot and then try to encourage lateral growth.

It will probably just shoot straight up if you keep it inside. Once outside, the higher light levels would encourage more branching (along with removing a part of the leader). But also, seedlings are known for being very vertical plants. Your plant is still very young. Maybe 5 to 10 years away from blooming yet.

Your seedling not producing any side branches at the height shown in the picture is very very common. The very top node on your Lime contains the dominant bud. The dominant bud restricts all lower buds from growing. If you cut off the top inch of the tree and put it outside as Dave in NoVA wrote, you tree will begin to develop side growth in a week or two.

Similar to what others have mentioned, you can put it outside if you don't have a brighter spot to keep it indoors, if you want to keep it indoors then you need to put it as close as possible to a west, south, or east facing window. Beware as seedlings that aren't accustomed to it can sunburn outside if you don't give them time to adjust. They can also do so in a bright window. If you've clipped it a couple times and it still won't produce side branches you need to cut it back further. Another option would be to wrap the main stem in soft wire (aluminum or copper, just make sure the aluminum wire isn't bare), and then bend it over to the side as far as it will go without breaking. This breaks apical dominance and encourages branching in a few weeks without requiring that you cut the tree that far back. (Look up wiring bonsai trees for the technique).