Need optimising in data base

I have adult classifieds on bad script.
.:Need to look into optimizing the database so that the queries complete more quickly, this can be done by creating additional indexes to improve the speed of searches.
queries are locking up causing my site to continue to shut down 3/4 times a day. Suggested by hosting, some kind of caching or open to suggestions. script is on php.
Thank you,
Carrie