Catalogs of ready-made challenges are usually split by format and by theme. Format most often means choosing between "solo" and "together with others": the second option looks more appealing thanks to the community angle, but on many services, iBro included, it is currently only partially available or still in development, so it is not worth counting on as the main reason to pick one right now.
Theme determines exactly which skill or habit you are training — from physical activity to reading or cutting out a bad behavior. Each theme usually contains several challenges of different lengths: from a short week to test the format to a month for those who already know they can hold on longer. There is no need to jump straight to the longest option — a short challenge finished in full builds more confidence than a month-long one abandoned halfway.
It is also worth looking at how cards are displayed visually: some catalogs show not just the gist of the task but also the shape of the eventual reward — a medal, a badge, a constellation — depending on the challenge's length. That helps you grasp what you are signing up for before you even start.
A useful detail when choosing: look not just at the current catalog, but at what will appear there later. Some services show upcoming challenges locked in advance, so you know what to expect in the next update. That helps you avoid grabbing the first option that shows up and instead pick a theme that genuinely resonates.