The first place to start is with phonemes: the 44 basic 'sounds' of English. Focus on the ones which are different from your own language. For example, you'll probably need to work on 'v' and 'w' sounds, and on long vowels and diphthongs ( e.g. the difference between the words 'not' and 'note'). You don't necessarily need to pay for a teacher at this stage. There are plenty of videos online teaching pronunciation - just check out youtube. Try practising minimal pairs: for example, train your ear to hear the difference between the teacher saying 'wine' and 'vine' and then train your muscles to produce the different sounds. These are all things that you can do without a teacher. When you've mastered the individual sounds, you can move on to word stress (including weakened syllables) and then phrase and sentence stress and intonation. As for feedback, why don't you record yourself and post a link here? Learners often do this, and italki members give comments and tips. All for free!

Su.Ki. gave excellent advice. The foundation is to be able to hear (distinguish) the 44 sounds (phonemes) of English. As a Farsi/Persian speaker, you will probably have some difficulty with the difference between /w/ and /v/ and the differences between the vowels (because English has more than twice as many vowels). Speaking accurately is difficult without feedback. Youtube videos provide knowledge about how to make the sounds of English, but most people speak with a strong accent unless they can get feedback. Speaking "without an accent" is nearly impossible (and unimportant). Being understood is the requirement in today's "global" environment. Speaking with "mostly correct" grammar, vocabulary, and pronunciation is the normal situation today when a Korean engineer talks to a French project manager in a video conference with engineers from Spain, Vietnam, and other countries.