[Suicide Junkie]

In stock, all mounts are available from the start, although in mods, mounts can require certain techs to be researched first.

Stock mounts require a sufficiently large ship hull before they can be used. Large mounts require a 400kt hull, heavy requires 800kt, and massive requires a baseship.

In stock, the mounts have tremendous damage/kt/turn increases with no penalties, so they are absolutely essential to use.
Most mods make a more balanced use of mounts.

In the design screen, there is a button on the top right labeled weapon mount; press that to select which mount you would like to use.
Any applicable components will then show the modified stats, and can be added to your ship.

Note that mounts do not have to always apply to weapons alone. Many mods use them to scale the stats of armor, engines and more.