How do I keep players from accessing certain commands? [closed]

By default, the majority of commands (with the exception of /me, /tell, /help, and /trigger for some reason) are restricted to server ops. Players are, by default, not allowed to use any other commands on a server. Server ops can use other commands. You can specify ops in the ops.txt file, or by using the command /op <playername> in the console, or in-game (assuming you have added yourself as an op).

By default, server ops have access to all commands, including commands like /stop. You can change this by modifying the op-permission-level in your server.properties file. Setting it to 4 (which is the default) allows ops to use all commands. Setting it to 3 restricts the /stop command, setting it to 2 restricts server moderation commands (/kick, /ban, /op, /deop), and setting it to 1 removes all special permissions from ops other than the ability to bypass spawn protection. You can still use all of these commands in the server console.

Adding on to @Unionhawk's answer you could aternatively just use a permissions plugin such as Group Manager or PermissionsEX with Bukkit along with the Essentials plugins. I'd definitely recommend reading some guides on running a Minecraft server, this not the best guide but a guide nonetheless and should point you in the right direction. Just google around to try find a guide that will suit you best in your objective.