Often I find difficult to write and mainly speak english because i don't know how to combine the right words to assemble a phrase. I know which there are many synonyms and my difficulty is how to find the appropriate word to match. What can I do about it?
Write and speak
one way to be certain about the usage of the phrases is collecting them together when learning them. when you have your own dictionary where you gather your words, you shouldn't just write words but phrases where they can be used as well, e.g. potato, couch potato, jacket potato, fried potatoes, etc.
another way to memorize phrases is read a lot and watch a lot. thus you can see the difference between written and spoken english.
what i do when i start a book is put a post-it paper inside it, and when i encounter a new word/phrase, i write them down. same happens when i watch something. then you can carry these lists with you everywhere and have a look at them anytime.
oh, and another way to remember phrases, or phrasal verbs - make wordspots or word 'spiders', like this: