How to clean startup on Android ?

There are times too many apps running on the background to slow down even halt the phone, I've to do a reset hopefully to clear the memory to start up clean, but to find that those apps I was running still exist after reboot, how do I stop them from auto starting ?

There are several ways, like:
1. factory reset, this will remove everything
2. uninstall unneeded apps
3. freeze apps (probably you need a rooted phone?), but usually it is made automatically
4. run the built-in cleanup feature

Without additional details hard to say more, probably there is no way.

...or just use ADB for non-rooted device