One misconception stems from the idea that any lwa will respond to a call. That's not entirely untrue: a lwa will generally come in response to a call from an asogwe initiate who knows the appropriate ceremonial langaj. But yr. average person in the street can't just draw a vévé, sprinkle some libations on the ground, then say "Hello,
There is also the risk of a trickster or lying spirit stepping up to the plate and saying "Hello, I'm a lwa and I want you to feed me." This risk may be compounded if you're taking the silly but not uncommon approach of "whoa, this one comes when you set live animals on fire and it eats broken glass, then destroys your enemies body and soul! COOL!!!" If you are asking for a fierce murderous spirit to do fierce murderous work, you may get what you asked for. But if you're not up to controlling that spirit, there's no reason to expect that it will be honest with you or take orders from you.
Sometimes the spirits you want to work with are not the spirits you need to be working with. If you are a rough and tough guy who likes to get in barfights, you may think Ogou would be an ideal ally and drinking buddy - but you may actually do better with the cooling influence of a Damballah or Agwe. You may think that because you love pretty, frilly things you want to work with Freda, when you might do better getting practical hard-headed advice from Danto. It's easy to make a mistake that leaves you even more imbalanced and out of control than you were before.
There are several ways to determine the spirits who "walk with you." One of them is by attending a fet and seeing what spirits wish to communicate with you or even try to possess you. You can also have a spirit follow you home if you attend a fet, visit a hounfour, or otherwise come in contact with that spirit. You may have your spirits reveal themselves in dreams or through omens. You may approach Legba, the old man at the crossroads: since he is the gatekeeper between this world and the world of spirit, everybody can approach him. You can also work with the Ghede: since we all have ancestors and all must die, we all have a connection to the spirits of the dead. (That being said, I would take precautions before doing this -- more below). The most reliable way of doing this is to get a reading from a professional who can tell you which spirits walk with you and who will work with you if you serve them.
Another thing which is important for anyone looking for their lwa is to make sure you have the proper protections in place. If you don't know how to banish and shield, you should learn how to do so now. I recommend the Vodou approach of working with the appropriate holy symbols. A blessed rosary around your neck and a blessed crucifix over your bed can turn aside most spiritual attackers and ne'er-do-wells. Those who aren't Christian can still call on St. Michael the Archangel, who is happy to slap spiritual nasties silly. (It's one of his core competencies)
Taking a balanced, slow path toward meeting your spirits and working with them will give you better results in the short and long term. There's no rush to get to Gineh: rather than calling on every spirit you can find, you will do better to figure out which spirits are interested in working with you and concentrate on them.