The first two attachments you unlock will be the Shared Silencer and the Reflex Sight. Once unlocked, start a game as a Medic and press to bring up the weapon attachments screen, now attach the silencer with and the scope with for the achievement. Attachments are unlocked by earning Vendor points (VP) which are earned by completing co-op missions and versus matches, similar to XP but at a slight slower pace.
Once you unlock an attachment, they are available to you straight away, you do not have to buy them. You can check the progression towards your current attachment unlock by going into the Vendor menu either while in a lobby or on the Profile screen accessed from the Main Menu. Then press to enter the Vendor catalogue, and press twice for equipment. You will also see your progress when you earn the VP at the end of missions and matches.