>>1773The weight you start with is whatever you can pick up and maintain proper form with for the duration of the exercise in question (which varies like
>>1778 said), NEVER start out with something you have to bend and twist awkwardly to complete a rep. Form is everything especially when starting out, if you get into a habit of poor form you WILL regret it later. Once you're sure you have the form down then just slowly increase the weight you're lifting every week.
As a general rule of thumb
most exercises aren't going to involve quick twitchy movements during a rep. There's some exceptions but usually you want slow controlled movement during a rep. Check this shit out if you want visuals to follow along with:
https://exrx.net/