A good way to analyze primary sources like this is by using the SOAPSTone method. SOAPSTone is an acronym for Speaker, Occasion, Audience, Purpose, Subject, and Tone.
The speaker of this letter is Private H. Baird. He appears to be a Canadian soldier in World War I. From what we can tell from this letter, he is still in Canada having recently enlisted and gone through basic training.
The occasion refers to the time and place of this document. From the date at the top, we can see that it was written in March 1915. This is a little over six months since the war began. Private Baird is still in Canada, but it seems like he will be sent overseas soon. It appears that his camp was in Rockliffe, but he is currently in Ottawa.
His audience is his sister. Private Baird addresses it simply with "Dear Sister," so we do not know her name. We can assume that the author of this letter felt that his sister might be concerned about his well-being.
From the start, Private Baird says that the purpose of this letter is to let his sister know that he is still alive. He wants to assure her that he is healthy and enjoying the soldier's life thus far. He also wants to send her a photograph of himself with this letter. He lets her know that he has heard that someone they know has been injured in combat. He says that he has visited "the folks" at home. This may be their parents, but this isn't clear. Overall, it seems that the writer's intent is to let his sister know what he has been up to and that he is doing okay.
The subject of this document refers to the overall background. It is still the early months of the war. Many Canadians have joined the armed forces and are on the verge of being sent in to combat overseas.
The tone of this piece is cordial but curt. The writer addresses his sister in a friendly way but does not go into much detail or explain his feelings at all.