Help - Seeking investment guidelines

Hi all, i'm new to investment and am looking for some help to guide me through a few tangles, all advice is really appreciated.

I have just finished uni and HSBC are allowing me to have an interest free overdraft of 1500, an offer i thought I shouldnt miss out on seeing as though I also have a job. As I am with HSBC i was looking at there savings accounts and wonder how wrong my calculations are hopefully any advice you have on how to invest some money going forward.

Firstly, HSBC offer a savings account with an 8% interest rate (gross AER - exactly what does this mean?). Assuming I put in 250 each month for 12 months, i calculate that I wound end up with somewhere in the region of

5753.36 after 12 months, or profit of approx 2753. Is this correct? Surely it can't be. What mistake have I made. Link Secondly, HSBC offer on online saver account with 4.65% gross, 4.75% AER variable. If I put the entire 1500 into that account, over 12 months I accrue 2588, a profit of 1088. Again, is this correct? Link I understand tax has to also be paid on these earnings, could someone clarify what this would be? Also, are there better alternatives and offers out there? Im new to all this and such advice would really be appreciated. finally, they state the interest rates are variable, how likely are they to fall into something terrible and un-worthwhile?

I appreciate the time taken to read this, its a long post! :D But all advice would really be appreciated.

Many thanks, Taz (watch out for the spam trap in my email address).