～ている means "am currently doing" (Think v+ing in English)
Dictionary form is more general.
So in answer to your questions:
今食べています is I'm eating (literally in the act of doing). 今食べます works fine grammatically but it has a different meaning. If you were asked when you were going to eat, you could reply with 今食べます which would mean you are going to start right now.
For speaking English, the examples would be more natural using 話す。
英語を話します = I speak English. (As with in English, you're able to speak English, but maybe aren't specifically doing it right now).
英語を話しています = I am speaking English. (The words coming out of your mouth, right now, are English... and hopefully they sound that way). :)
There are some pitfalls where your English speaking brain will want to use one over the other, but it's more natural to use the reverse. Don't worry about that for now though.