clench/grasp/grip/hold HOLD is most often used. e.g: I'm holding a cup. She held the boy. Can you hold this? I feel as though GRIP is often used when talking about things that may be safe/unsafe or as part of a procedure. e.g: He lost grip of the rocks as he was rock climbing. You need to grip the power tool here to maintain control. CLENCH: This is usually used to say "she got so angry, she clenched her fist" meaning that her fingers closed into a ball. GRASP: Not often used. means to hold tightly. "She grasped her water bottle as she walked over the bridge"

"Clench" has quite a narrow range of uses compared with the others, which are more or less synonymous; however, they cannot always naturally substitute for each other. You could study example sentences which you can see in dictionaries and sentence databases like fraze.it. Once you have some idea of suitable contexts, you can then experiment by creating your own sentences and posting them for correction. I would start with "hold" as it is the most common word, and has the widest meaning.