I have a Suzuki Boulevard S50 and i have changed the solenoid because i thought that was the problem but it wasnt. every time i hit the switch to start the bike all the lights dim but the bike wont start

The starter button is supposed to turn off the lights during the start. The new solenoid means that is OK. That means the starter is the problem. Tap on the starter with a plastic hammer, back of a screwdriver, etc., if the starter works now you just need new starter brushes. If the armature is shorted, the end cap bearings worn, the magnets broken, etc. then you will need to replace the starter assembly. Only the brushes are available as spare parts to fix the starter.

Short the starter solenoid positive and negative terminals; if bike starts, problem IS NOT starter.