I recommend the (below) site from the University of Iowa. You can use the app on a PC (but not a Mac — it uses Java or something), or get the mobile app:

http://soundsofspeech.uiowa.edu

That being said, if your problem is with “short words” such as the ones you’ve mentioned, the issue is with the use of so-called reductions. These unstressed words are all pronounced with the schwa (reduced vowel) in normal conversation, regardless of the vowel that is used when they are stressed. The IPA symbol is /ə/.

Standard US pronunciation when emphasized:

are /ɑːr/

you /i̯ʊu/

or /ɔr/

to /tʊu/

Standard US pronunciation when reduced (i.e. most of the time):

are /ər/

you /i̯ə/

or /ər/

to /tə/

It’s important to realize that the reduced vowel (“schwa”), although it doesn’t have its own letter, is the most common sound in the entire English language. Although native speakers will still understand you without the schwa, you’ll sound odd. More significantly, if you haven’t trained to listen for the schwa, your listening comprehension will be an uphill battle. On the other hand, mastering the most common sound in the English language will open up your ear very quickly.

Btw, do not trust Forvo, because native speakers on that site often use an artificially “correct” (i.e. false) pronunciation that is never used in real life.

Edit: I corrected a typo which autocorrected incorrectly and could have been confusing to learners. I also re-worded a part that seemed redundant.

Thank you all. Your advice is useful.

http://www.fonetiks.org/

It is free, and it gives audio recordings for almost all the vowel pronunciations as well as the more complicated consonant sounds in the English language! 

All it really takes is a lot of practice, by listening and repeating words or phrases that you see online. I recommend listening to podcasts, or music (which will really help get that American accent down) on your free time and practice on your own. 

https://youtu.be/-SObetOJ_yY

This link is a video that shows you the mouth placements for all of the pronunciations I could think of!

hope this helps!

I'm happy that you found our advice useful, Carlie :)

I recommend you to listen to a lot of English based materials (like TV shows, movies, news, ...etc) and here the words from native speakers that way you'll improve your spoken English very well.

If you are looking for a course here is the one I have and used for American accent very good and well taught. 

http://www.pronunciationworkshop.com/ ;