The strike price of an option is the price at which the contract can be exercised. The strike price of a stock and an index option is fixed in the contract. Depending on the amount of premium you want to spend, you may want to set the strike price higher or lower. Generally, if you are buying call options, a higher strike price results in a cheaper option and vice versa for put options. Setting a strike price depends on the amount of risk you want to take and how much you are willing to spend on purchasing the options.
If you buy or hold a call option, you have the right to purchase stock shares at the predetermined strike price. For example, suppose you want to purchase at the money call options of stock ABC and the stock price is currently trading at $20. Since you want to purchase at the money call options, you would set a strike price of $20. This indicates that if the stock stays above $20 before the expiration date of your call options, you could exercise your options and buy shares of ABC for $20.
On the other hand, if you buy or hold a put option, you have the right to sell stock shares at a predetermined strike price. For example, suppose you are bearish on company DEF and think that it will trade below $50 in three months. Company DEF's stock price is currently trading at $70. You could purchase put options and select a strike price between $50 to $70 depending on your risk tolerance.