Website Performance analysis

An ecommerce website build with opencart. It has been customized and added lot of queries using core php scripts. Queries added directly to template, create separate modules in core php to manage orders and products. Now the website becomes too slow. The server management team updated that lot of requests coming from various part of website to the database, it affect the performance.

How can i optimize the query performance of website ?

Also I want to optimize mysql table performance

Please anyone help me to suggest the solution