At first Decide why you want to buy a smartphone? To competition your friend or want to play game or want to show your status.After your decide then note it and decide your budget. Please do not cross you budget limit for buying a smartphone. After this work you can follow our tips
Processor

At first buying a phone check the smartphone feature. Here you think what is Processor? Processor is a mind of the smartphone. Various company made the processor here the basically 3 popular company made processor Snapdragon, Mediatek, Samsung. Then check it which type of processor in this phone? Dual core, Quad core, Octa core. Here you think what is the core? We are eating in a mouth. If you have 2 mouths you process the food quickly if you have 4 mouths you process the food more quickly. Same concept in Dual, Quad, and Octa core. Dual core means 2 mouths Quad core means 4 mouths and Octa core means 8 mouths. A processor have many transister. More transister means more better processor. ARM set the size limit of processor so company try to making best processor they put the more transister so they are cutting the size of transister few days before transister size was 26nm then come 12nm today come 10nm. So check the transister size of processor. Best Processor give best Battery performence. Then pick up some processor in your budget and check in google which is better Processor.
BatteryAfter choose the Processor choose best battery. Battery is giving energy in your phone. To buying a smartphone shop owner ask you which battery you want 3000 mAh, 4000 mAh or 5000 mAh.Now you tell me what is mAh? mAh mean-milli-ampere hour. Your phone has 3000 mAh battery and it is consume the battery 300mAh/hour here your phone battery extended time is 10 hours. So choose the best battery in your phones capability. If your phones 6 inch display and dual core processor and your battery is 4000 mAh. So we are tell you your phones battery extend in only 3-4 hours, But your phones have Octa Core processor and display is 6.5 inch and battery 3500 mAh here your phones battery extended time is 9-10 hours so check your phone capability and check which brand giving more mAh battery.

Screen and Display

After select the processor and Battery then select display. You choose the display size in your phone and follow the tips
 First check your phones display have Gorila glass. Gorila glass protect your scree from scratch. In 2005 corning inc made gorila glass they first made gorila glass 1 then they decide cut the thickness level and launch the gorila glass 2 and 3. After made gorila glass 3 they have research and check the 70% phone screen scratch from fall into the hand and they made gorila glass 4 and 5. Gorila glass 5 is available on above 30000 rupees smartphone. So check your phone which gorila glass is available on your smartphone. Then you decide which display you want bezzel or bezzel less. 
The Next step is check your phone resolution your phones must have hd display if your phone have no full hd display than what is the benefit to buy a phone. Do not buy 720p resolution display buy hd Display.

Camera

If you want to Buy a phone for camera then first check phones picture quality because the branded smartphone 5 mp and china smartphone 8 mp have do same work first check the picture qualiy personally. Then check how megapixel(mp) 5, 8, 13 mp. If you want to buy phone only camera you can buy camera because phone special is for calling.

RAM

RAM means-Random Access Memory. Ram is very important to a smartphone before buying phone check the phone ram. Minimum 3-4 GB Ram is sufficient in a smartphone.
Network and Other

First check the sim network there check your phones sim card support dual 4g+volte network or not then check fingerprient scanner, waterproof 3.5 mm headphone jack etc. and confirm it that how much software update coming in your smartphone more software update mean more better phone.
In today is not available all best feature in a smartphone and if you want to buy a latest version phone than forgot it because the company update the phone after 3-4 months.