You can add half your Proficiency Bonus (round down) to any ability check you make that uses a skill proficiency you lack and that doesn’t otherwise use your Proficiency Bonus.
For example, if you make a Strength (Athletics) check and lack Athletics proficiency, you can add half your Proficiency Bonus to the check.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.