I'm 26 and having a slight pain in my far left chest when lifting arm and when sneezing once in a while it's tender?

Muscular: You could have muscular strain / sprain or inflammation - probably the pectoral and intercostal muscles. It is also possible that you have inflammation in the lining of the lung which is called pleurisy. NSAIDs will help either condition. If you have worsening pain, severe shortness of breath, fever or fast heart rate - see a doctor.