Yes and no, you don't exactly summon a deity, frankly as Lewis said it's a bit foolish and rather rude which means they're probably going to be less inclined to help you if they attend, you ask for their help. Think of it a little like asking a friend or relative for help, you don't order them to help you but rather ask nicely.
If you want to ask a gods advice or favor then I'd suggest that some form of prayer (all religions have at least an equivalent for talking to the deity/deities of their faith). In times when you particularly need your gods' advice then you invite them to attend and help you however bare in mind that only the gods you worship relatively frequently are likely to help, just in the same way that a friend would help you where a total stranger would likely blow you off.
Offerings vary among practitioners and deities so it's almost impossible to say that you should offer them X, Y or Z but rather think about it and the situation. Try to think of a gift that the deity would appreciate and something that feels relatively heart felt, an offering is your way of thanking the deity for their help so frankly the more personal to the relationship you share the better.