1995 Honda Civic PLEASE HELP:)

1995 Honda Civic Manual 265000 miles

hi there, just a problem ive been having with my car. Sometimes but often when I am stopped at a red light or just stopped in general my rmps rev between 1000,2000. Its a constant rev like goes up and down up and down. But then sometimes its completely fine just wondering what it could be? Ive recently put oil in it and filled it right up with gas. I do need an oil change, but I really dont think thats it. A friend told me it might be an oxygen sensor needing cleaning. But I really dont know, and since im a girl I dont want mechanics to rob me money! Haha please help thanks for your time:)