There are common Bicycle Assembly Mistakes to Avoid, top 4 Mistakes People Make Assembling Their New Bike:

1.Why the handlebars/stem keeps wobble? When you try to turn left or right, the handle and wheel go on the other side.

A: Top one of common mistakes, Rotate the fork and head-stem, so they are facing the correct direction, then align the stem with the front wheel and tighten all bolts as shown on pictures what we sent.
Test your work to see if they move sideways. If so, re-center and tighten them until they hold fast.

2. Q:  How to tell which pedal is which? 

Why my pedals are bent or doesn’t go well?

A: Look for “L” and “R” marking on axle or wrench flats. If no “L” and “R” marking are seen, use pedal thread direction to identify pedals. Left threaded pedals (threads sloping upward to the left) go to left crank. Right threaded pedals (threads sloping upward to the right) go to right crank.

3. Q. Why brakes struggle to stop the bike? And How to get rid of pesky rubbing disc-brake?

3. Q. Why brakes struggle to stop the bike? And How to get rid of pesky rubbing disc-brake?
A: To do this, loosen the anchor bolt, grasp the cable, move on up the cable and re-tighten the anchor bolt.

Alignment procedure is the same for front and rear brakes, loosing the mount bolts until the caliper body is freely side to side. Now we squeeze the brake lever to center the caliper body over the rotor. While holding the lever, snug up the bolts.
If there no rubbing, the pads are aligned, secure the mount bolts to full torque and your work is complete. 

4. How to adjust your front and rear derailleurs?

Smooth bike gears can make all the difference to the enjoyment. Although they seem complicated, getting your gears running properly is actually quite simple. 

Adjusting Front Derailleur 

With both shifters set to the smallest number, disconnect the cable from the anchor bolt and place the chain on the corresponding sprocket and chain wheel. The outer plate of the front derailleur cage should be parallel to the chainwheels, and should clear the chain wheel by one to three millimeters.  

Adjust the low-limit screw so the chain is approximately 2 mm from the inner plate of the front derailleur cage. Pull the cable taut, reconnect it, and tighten the cable anchor bolt. Now while turning the pedals, shift the chain onto the largest chainwheel in the front, and onto the smallest sprocket in the back. 

If the chain doesn’t go onto the largest chain wheel, turn the high-limit screw counterclockwise in 1/4 turn increments until the chain engages. If the chain falls off, turn the high-limit screw clockwise in 1/4 turn increments until the chain no longer falls off but does not rub on the outer plate of the derailleur cage. The outer plate should be about 1 mm from the chain. 

How To Adjust a Rear Derailleur on Your Mountain or Road Bike

While today’s indexed shifting can make shifting nearly effortless, when problems arise it can be annoying or even dangerous. A sticky, grinding or a jumping chain means it is time to inspect and adjust your rear derailleur. This job is best done after chain has been cleaned and lubricated.

Steps to fix a miss-shifting derailleur:

1. First, inspect cable and housing to make sure they are not rusted, split, or cracked. If the cable does not move freely then watch video on How to Change Rear Derailleur Cable. Next, inspect chain for stiff links or bends by running chain backwards to see if it jumps. If jumpy, sticky or skipping, then you can try flexing back and forth to free. If links remain sticky, then check out another video on chains for instructions on how to fix.
2. Next, If cable and chain are good, check derailleur cage alignment visually from behind the bike. It should be parallel to rear plane frame. If parallel, then you are ready to set H stop screw. If not dial in with H screw until aligned.

Setting H adjustment screw:

1. Shift derailleur to highest gear or most outboard.
2. Look at pulley cage if it is too far outboard turn screw.
3. If too far in turn (H) one turn counter clockwise.
4. If not then check cable tension with barrel.
5. Try shifting to next cog up if it makes the shift quietly, then skip ahead to setting ( L) screw.
6. If not, then turn barrel adj to align with second cog.
7. If good but noisy then fine tune tension with barrel adjuster to fine tune.
8. Repeat

Setting low gear stop (L)screw:
Setting the upper travel limit for lowest gear is particularly important as improper adjustment can result in a bent derailleur cage, spoke damage, and a sudden stop on an uphill grade. Sometimes, all of the aforementioned can occur which can be dangerous and expensive. To set:

1. Move chain to lowest gear (largest cog near the spokes) and then check cage to see if it is directly under with cog.
2. Find L stop screw and turn until it just moves and then back off 1/16 of a turn

Once you have done that, then shift up and down the gears while pedaling with your hand noting where any fine tuning is still necessary. Fine tuning can be accomplished through barrel adjusters on the shifters, derailleur body and on the downtube mounted barrel adjusters for road bicycles.