Just to comment Esther's answer about the use of "un".
At least you say: "Soy un ingeniero como muchos otros" / "I'm an engineer as many others" or "Soy un ingeniero que cambiará el mundo" / "I'm the engineer that will rock the world" then "un" is completely unnecesary. In other words, If you are just going to tell your profession , the "un" should not be used. But, If you are going to use your profession to make a sentence, an exclamation or an statement, then it is mandatory.
For "toma asiento", it's like Esther said, since your are supossed to take one seat we don't use "un". But if it's necessary to remark to take just "one" you can say "tome un asiento", "coge un cigarrillo", "toma un lápiz". Sometimes "un" is necessary to be formal, Maybe it will happen that you think the other person will take advantage of the situation.
Also sometimes we use "un" because otherwise it will not sound good. For instance, "toma un vaso" (de cerveza) / "Have a drink". You cannot say "toma vaso" because that doesn't make sense. You can say "toma un vaso" not because you want the other person to have just one, but to be gentle, and because you normally say it that way.