https://strengthlevel.com/one-rep-max-calculator You can also use this to calculate your max. Its not going to be exact but it will give you a good idea.
http://liamrosen.com/fitness.html pretty good intro link from /fit/.
Its extremely important to learn proper form when lifting otherwise you risk injury and low gains. Also I would highly recommend not to max out until you have your form down especially for dead lift and squat. Back injuries...