There's many reasons why your site can be slow. But when using Gtmetrix, take a look at the Waterfall section first. This will usually give you a very good idea of what the main culprit is. Click on the + sign to get details.
Then use the Pagespeed and Yslow tabs and expand the recommendations to get details and implement any suggestions that you can.
Keep in mind that there are some suggestions that you have no control over so don't waste time on trying to fix them.
Main culprit usually is images so check those first and get rid of any plugin that is actively slowing down your page load speed.