The three main variables you have when choosing a guitar pick are thickness, material and shape. These can all affect your comfort in playing and the tone of your instrument.
There are some general guidelines you can use when choosing a pick but it all comes down to personal preference in the end. Many musicians do the opposite of these rules, so always take "rules" with a grain of salt!
The general guidelines are that for strumming you want a medium to thin pick and for soloing you want a medium to thick pick. So by that logic, a medium pick is a good general purpose pick for both strumming and soloing. But I'd like to point out, I've seen musicians who actually prefer thin picks for everything and some who prefer thick picks for everything. So again, never follow guidelines as gospel!
This can impact the tone of your guitar in a pretty dramatic way sometimes. A softer material like nylon will mellow your tone, while a celluloid pick will not mellow out as much of the higher frequencies, and a tortex will be even brighter sounding than both of those. So it really depends on how bright sounding you want your instrument to be. This is where the type of strings you use can also play a part in your pick preference because the pick and strings come into direct contact. If you are someone who doesn't really care about tonal differences and just wants a pick that's easy to grip, the pick material can have an impact on grip-ability as well.
Most picks come in the classic "teardrop" shape but pick shape is a highly personal preference thing. Some find that a smaller sized teardrop shape could give more precision but in general, the classic teardrop shape is a good all around choice for most musicians. There are also triangle shape picks, which are a little less common.
At the end of the day, making music that inspires you is the most important thing and your guitar pick is just a simple tool that's often overlooked by many. So to sum things up, there is no right or wrong pick because every musician is unique and different. But as grandma always said... "Yup, we're all different. The world would be pretty boring if we were all the same."
Thanks for reading, and I hope that if you do decide to explore new guitar picks, Pick Your Picks gives you a simple, cost effective, way to do so. Happy picking!