dekh = see, dekh lena = see, so wht's the difference of those ?

this one will be a little too much for a beginner because i believe here comes subtlities of Hindi

Dekhna देखना- to see
Dekh lena देख लेना- may also mean to see, but there is difference, also dekh lena may mean some other things too.

they also are not interchangable many times except for the case where it means 'to see'

for e.g.

dekh lo, agar aana ho to. (think about it, if u want to come)
देख लो, अगर आना हो तो.
here dekh lo means think about it/or have a thought.

dekh lena may also mean to examine/inspect

sir, copies dekh lo

take another case

जो दिख नहीं रहा, उसे भी देख लेना दिव्य दृष्टि कहलाएगी।
which is not visible, seeing(to be able to see) that too will be called divya drishti.

here 'dekh lena' means 'dekh paana' i.e. to be able to see.

last i can remember

dekh lena is also used to indiacate holding grudge against someone e.g.

tujhe to main dekh loonga- literally 'i will see you' :D

i have missed some cases, someone might add them up, and i am totally dissatisfied with this answer because still sentences are popping in mind and i m unable to articulate them properly.