Notes:
1. Put a drop of light oil on each nipple where the spoke enters it, and spin the wheel to get the oil to work into the threads.
2. The 'heavy duty' steel wheels on the Murray are only there because they are cheaper than alloy rims. They are weaker and heavier and more difficult to fix than basic alloy wheels. They also provide almost zero braking when you try to stop in wet conditions because of the chrome finish..
3. The saddle on the Murray is too low for the person sitting on it.
4. An out-of-true rim is normally very difficult to feel when riding. If you feel a 'wiggle' as you ride it is more likely caused by a damaged tire casing or tire not fully seated on the rim.
5. All other advice given here is correct. I use a zip tie around my seatstay to true my wheels. I remove the wheel and install it backwards and see if the zip tie is at the same place relative to the opposite side of the rim - this is how I check 'dish' (how centred the rim is between the outer lock nuts on the hub) without a single purpose 'dishing tool'.