How to download compatible APK with phone?

[AC Question]

My question is very simple but may be you have to face trouble understanding my bad English..
Well I am new to android, I just want to know that what to look when download any APK, I mean I have Droid Razr M, & I want to download Whatsapp so which version should I download ???
Should I download latest version or choose any from older versions??
I want to know which version will run perfectly on Razr M ??
Just like that I will download all social apps so how will I know that this version of Facebook is good for my phone and that version of viber is good ??
How to know????
Answer Please
Thank you

 

[the tall guy]

Hi, if you're looking to install quite a few apps why don't you just use the play store? Apk files are good if you want the lasted version that isn't available on the play store yet.