How to used to using guitar picks?

First, be aware that pick choice is even more individual than string choice. If you surveyed 100 top players, you'd find dozens of different types of picks.

In general.... Thinner picks are often used for "strumming", where few if any single notes are played.

If you are playing a lot of single-note runs AND strumming, like is done in country or bluegrass playing, you'll want a medium or medium/heavy pick.

For lead playing, playing only single-note lines and such, then a heavier pick will be better. Most jazz players use very heavy picks indeed, often 1mm and thicker.

Techniques include simple up-and-down strumming, playing single bass notes or connecting runs and strumming also... And playing fast single-note lines by alternate picking.

How to become proficient? Practice.