There are many weapon options in MGO, but depending on if the match is using Drebin Points or other host options, some weapons might not be available. Without Drebin Points, the basic weapon is usually a AK102 or M4 assault rifle, pistol, and knife. Most matches have weapons like the Vz. 83, shotguns and rocket launchers deactivated because they're too easy for unskilled players to get kills with.
When Drebin Points are activated, you'll get points for how well you do during the match, which are redeemable at each respawn for more advanced weapons that aren't selectable otherwise. Again, the host has the option of what weapons to include, so don't be reliant on any particular weapon, although the M4 is almost always available with or without Drebin Points. Use the online training ground or MGS4's shooting range to get a feel for each weapon.