The Glaive is a melee weapon that you can get from two different ways. The first and the expensive one will be buying it with Platinum through the Market Console. If you buy it that way, you also acquire an installed Orokin Catalyst and a weapon slot in your inventory.
The second possible way will be completing an Alert Mission which offers as a reward the Glaive Blueprint. You need to stay tuned, Tenno, because Alert Missions are active only for a limited time, and you can not foresee when appear the one which has this weapon blueprint as a reward.
When you have the blueprint you will have to go the Foundry of your ship and craft it. This is the slowest way, but is the way most Tenno use to get the Glaive.