whats the last trading amount in trading stocks?

I have a doubt that when we order to buy shares without limit price itll get executed with the last traded price means every sec the price oscillate then how cum we can be sure that the shares are bought at the price that we looked for?Then please some one tell me about mutual funds basically i dont have any idea about it,
Id be thankful to you if u resolve these questions

thankyou…………………….