[Red sea shark]

Some knives come only with straps, others with everything you can imagine. If you only intend to strap it onto your leg, then don't get a dive knife with hose attachments! If you are unsure, then go for a dive knife with many options; the T-Rex knife (from Lumb Brothers) is a particularly good option.
When deciding where to put your dive knife make sure you can reach the dive knife - strapping your knife onto your leg is notorious for being difficult to reach. Another common place is on your arm - note that if one arm gets snagged you'd better hope it’s not the one you need!
The best place to put a dive knife in my experience is on your BCD.
Go for a titanium dive knife unless you only want it for a month or two, and get one that suits your needs - in many cases a no frills titanium BCD dive knife is perfect!

I hope this will help.