There isn't much you can do to increase your beast companion's attacks. This is unfortunate, but the good news is that it's going to get better.
At early levels, a beast companion's attack is going to stink. For whatever reason they gave it a static modifier that is going to put it way behind even the worst optimized characters meaning that probably from L1-L6. At L7 the beast companion catches up to a poorly optimized character (+4 to stat, +2 proficiency, no magic item or feat bonus). It will continue to roughly keep pace with PCs after that since it increases every level up and PCs only naturally increase every other level (though it's effectively every level with magic items, feats and other increases).
Unfortunately, this means that at early levels your bear is going to have a hard time hitting, though it does do solid damage when it hits (d12 is nothing to sneeze at). Unfortunately there isn't much that can be done to optimize it's to-hit bonus further. There aren't any feats that boost it's to-hit and none of the animal companion slot items boost it either.
However, there is a way to consistently get a +2 to attack. Flank.
There are lots of powers that make your animal companion a shifting fiend. Use them. Get your friend in position opposite yourself or your friend and let him gnaw on the monster in front of him.
All of this to point out that Beast Master is a suboptimal choice for a ranger. A better choice, if themes are on is the Fey Beast Tamer theme presented in Heroes of the Feywild. Another solid choice would be the Sentinel Druid which gets a much better version of a bear (hell, you might talk to your DM about just getting that version of the bear companion).