Metal atoms lose electrons and non-metal atoms gain electrons.
Metals are on the left side of the periodic table, Groups 1 and 2 and the transition elements, Groups 3-12. Metals lose electrons to achieve a stable octet, which means eight electrons in their outer level.
The elements in Group 1, the alkali metals, all have one valence electron which they give up easily in chemical reactions, forming +1 ions. Group 2 elements have two valence electrons, which they lose to form +2 ions. The positive ions that form are more stable than the neutral atoms because the next energy level down is complete. Most transition metals can lose various numbers of electrons and Aluminum in Group 13 behaves as a metal, losing three electrons.
The reason metals don't gain electrons to achieve an octet has to do with electrostatic attraction between the positively charged protons in the nucleus and the negatively charged electrons outside the nucleus. Sodium, for example, has 11 protons and 11 electrons when neutral. When it loses one electron the remaining 10 electrons are attracted to 11 protons. There's not enough nuclear charge to attract the seven additional electrons that would be needed to complete its octet. Non-metals only need to gain from one to three electrons for a complete octet so the charge difference is smaller.