I have a 2000 Nissan Xterra with 120,000 miles. On cold mornings the brake light on my dash stays on til my car is warmed up. Should i be worried about this??

My A/C has been making a hissing sound, it is still blowing cold air. Should i have it check. The last time this happened I just let it go and i ended up having to replace the hose which cost like 300.00. I had to replace the hose twice already. So i guess my question is will it pay me to have it check out now??

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The brake light is due to low brake fluid. When it warms it expands just enough to make the sensor happy. The fluid is almost certainly low due to brake pad wear, the caliper pistons sit further "out" of the cylinders and more fluid stays down in the cylinders instead of the reservoir. Check the pads for wear, add just enough FRESH fluid to keep the reservoir sensor happy, and only top off the fluid after the pads have been replaced or you could end up with a mess under the hood if the pad replacement isn't done without opening the bleeder screws to push out the old fluid. A proper job on a modern ABS-equipped vehicle should not push the fluid back up the line, but most shops don't bother.

As for the A/C, look for an oily leak somewhere to start with.

I changed my tires like 8 months ago and they checked it then. So do you think i should bring my car in to have the ac checked?

I don't know what to tell you there. It could be fine, it could last a week or a year, who knows. Only way to know if there's a problem is to get it checked.