always eat the first tower , buy items around 7 minute mark

That depends on the stage of the game. Almost any tower has the potential to become strong, but in earlier levels you should stick to some stuns + control, such as Zeus, Doctor, Pudge, Centaur and etc. A great initial decision would be to get a level 6 Shadow Fiend with aghanins, since his damage can escalate to the infinite. He can combo with Sven (with aghanins), Magnus, Ogre, Venge and etc. Take care on where you put towers that have auras, such as CM and Venge.

Later Sniper and Drow can outcome a high level of damage and stun/slow creatures that could escape, especially bosses. The fun is to experiment things and see what may work out

The funniest thing I did was to buy Pudge an aghanins. Definetly.