how do you get your lower back stronger?

i find when i do pushups my lower back starts to ache. any tips?

the only safe back strengthening exercises that spring to mind mate are dorsal raises. sort of backwards sit ups designed to strengthen your trunk. but make sure you check that you are performing them correctly before hand

dont you need some type of machine to do those? , since im a home trained guy maybe i could make something?

Have a look at the stretch thread theres some good ones for the back

Stretching and warming up is important and here's a tried and true method:
Bridging helps stretch your back and strengthen it all at the same time. What you do is lay on you back than arch yourself upard supporting yourself on your hands/head and feat. This a great effective exercise that's been in action over the years for one reason it works, and for a lot a reasons. In your case it'll take the load off your spine allowing it and the rest of your back to relax. You can do it with or without equipment.

One exercise I have found to helps work the lower back is knee/leg raises off a chin-up bar. This is one tough exercise it's one of the best exercises for the abs in conjuction with your back which acts as a stabiliser, this also takes a load off your spine letting you build much needed strength at the same time.

One way to get your lower back stronger is get a exercise ball and lay on your stomach on the ball, get your feet up against the bottome edge of the wall for support and stability and raise up like a backward situp. I know it was stated before but I just wanted to show it can be done on a ball with really good results.

You're right there. I'll be honest I got an exercise ball myself, I find it's great for stretching and working the muscles in different ways. Try doing leg-raises with while holding an exercise ball between your feet, it's a different experience entirely. I don't use it for the bridge, but none the less I like to use it to stretch my back when it's sore. Besides that it can be heaps of fun.