First and the foremost tip to buy a mobile phone is to set your budget for the phone first of all.
And then check out the specifications of the phones available in your budget .
Check all the specifications of the phone and compare prices if possible check online prices.
Also read reviews about the phone online in different websites and after then make your mind.
The most important thing go for good battery phones so that it can provide you good backup in emergency conditions also in the daily usage because no body likes inserting charger in the phone every hour . Also phone should have good connectivity .