Ok, let's see...
First of all, when you start out with a new phone, you're unrooted. You're at the mercy of the crap and gunk that comes installed with your phone. This is typically called "Bloatware." You're simply the owner of the phone and at the mercy of whatever your carrier (in this case, Straight Talk) decides to shovel on it. You also have to deal with the pathetic amount of internal memory that your phone has been installed with.
When you root your phone, you basically become the administrator of it. This has good and bad points. First of all, you're free to scrub out any software you know you'll never use and free up internal space and system memory. It also give you the ability to open up all sorts of perks (for instance, using Link2SD, which will give you SO much more space than that measly 150mb). The downside... if you're not careful, you can tank your phone. Rooting is very much a double edged sword. It is worth it on so many levels, but that freedom to do what you want with your device that you paid for and own can also turn it into a nice looking paperweight.
If you still want to make the leap, use the directions found here:
http://androidforums.com/ascend-2-all-things-root/641614-finally-rooted-m865c-straight-talk.html
For cleaning the browser cache, go to "Settings/Applications/Storage Use". Scoll down to "Browser." Press "Clear Data" and it will get rid of all all the junk it accumulates. Unfortunately, it'll also scrub passwords, history, etc...
I hope this helps a little. I'm still learning a lot myself, but I want to give back to the community that's helped me this far.