I've been speed cubing for 2 months now, using the Fridrich's method. I just got a sub-40 average like 2 weeks ago, and now I'm trying to get sub-30 average. I've been following a guide somewhere on this forum (I got the link from the wiki) to get a sub-15 average. I've learned 21 cases of PLL but still I can't get a sub-30 average.
Cross+F2L is around 20 seconds average, and the rest (around 15 seconds) for the last layer. So do I improve my F2L, my last layer, or I just need to get a DIY cube, because I'm currently using a cube I bought from Toys 'R Us, lubricated with silicon spray. It tends to get stuck when cutting corners, and it doesn't turn that fast

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i would recommend trying to persuade your parents to get you a DIY. internet shopping is very safe nowadays and i know of people who almost exclusively buy things off the internet. setting up a paypal account is also very easy to do, and the DIYs only cost about 8$ anyway

cheaper than storeboughts (at least in the UK)!
anyway, i would work on your LL recognition - getting a cross on top should be sub 3 at worst(maybe sub 4 if you have to do two algs), as it is very easy to recognise.
your second orientation stage should also be pretty quick - there are only 7 cases, and all of which are pretty fast.
all of your PLLs should be <5s at worst, and your recognition should only take a second or two.
for your f2l, do as many solves as you can to get familiar with all of the different cases. instead of solving with your usual 'method', when doing the f2l, try not to rotate, and look for the most optimal solution you can for each case.
the most important advice that anyone can give, for f2l, is GO SLOW, AND LOOK AHEAD. smooth and steady wins the race, at least when you're against someone who turns fast, but is sloppy and keeps pausing
with the cross, try and solve it blindfolded every time. doing this loads really helps with execution - you can just spend a couple of seconds memo'ing the cross, then execute the moves as fast as you can while looking for the first pair you want to start on. considering you should be doing around 7 moves, sub-3s cross becomes surprisingly easy
i know im essentially reiterating what everyone else has said, but oh well...
good luck.