>>33365776 (OP)The answer depends on whether you want to sound British or American. As I'm British, I'll focus on that.
The most important thing to work on is the difference between long and short vowels. For example, the difference between "rich" and "reach", "cot" and "caught", or "can" and "khan". In Romance languages there tend to be a much smaller range of vowel sounds. My partner, for example, is from Italy, and there are only seven vowel sounds in Italian; in British English (depending on the dialect) there can be as many as twenty distinct vowels, and distinguishing between long and short versions gives foreigners a lot of trouble. Practise saying "Who would know aught of art must learn, act, then take his ease" and then "my loud voice nears their moors" - that covers most of them!
Most vowel sounds in English are more open than Romance equivalents.
Also, be very mindful of the neutral vowel, which is the sound most other vowels tend to degenerate into if you're in a hurry :) or the vowel sound in "huh". It's a very common sound in English. For example, a word like "teacher", foreign speakers tend to render as "tea-chair," whereas it should be "tea-chuh".
That's also a good example of the fact that in RP British English, the letter "R" is generally silent, unless it comes before a vowel that is part of the same syllable, or at the end of a word but the next word begins with a vowel and you're eliding the two. So, again, the "r" at the end of "teacher" is silent; in the word "river", you pronounce the first "r" but the second is silent ("ri-vuh", with a short "i" and a neutral vowel). The English "r" is also quite different from other languages: further forward in the mouth.
Also, British English tends to have a greater range of different pitches than other languages. And of course there is a far looser connection between spelling and pronunciation than in most languages, so don't suckered into learning to pronounce words the wrong way from the spelling.