Unless you are willing to invest in a bunch of Natural Bond feats (Complete Adventurer) or Beastmaster levels, I'd second the above advice. Your companion will be far too weak to stand on the front line with you, so it needs some other role.
I'd suggest either a mount if you do a lot of traveling or outdoor adventures or perhaps a scout/spy (such as the owl).
If you are set on trying to make it a meleer, a Riding Dog is the toughest thing you can get. Teach it to "aid another" and help your attacks or your defense; perhaps you can also get it to flank.