"Lag" is a lazy term. To fix your connection, you need to identify the specific metric failing you.
1. Packet Loss (The Network is Failing)
Data sent from your PC never reached the server.
Symptoms: Enemy models freeze, then teleport. Your shots don't register.
Causes: Bad Wi-Fi signal, congested ISP node, or a damaged cable.
2. Choke (Your CPU/Bandwidth is Failing)
Choke happens when your PC or the Server is trying to send more updates than the connection allows.
Fix: Adjust your game's cl_updaterate and rate commands.
3. How to Diagnose
Use the Command Line:
pathping 8.8.8.8
Pathping analyzes every hop between you and the destination for 100 seconds.