How to regulate the temperature ??

I judge by looking into the top vent without the daisy on. I use propane and light between one and four places on the lump depending on how hot a fire I want. Doing a low slow I would light in two places. After a few minutes I peek down the hole. Hopefully I would see a two red glows where I lit. I then set up platesetter grills and whatever, put on the Daisy and start shutting down when temp was getting close.
Temp will drop a bit when you put the meat on.

 
First off congratulations on your new egg and welcome to the forum.

Getting your lump going well then adjust usually runs into problems like you are having. Begin adjusting your lower vent when you are about 20° - 40° below your target cook temperature then continue fine adjusting as you begin to reach your target temp.

The temperature can be controlled by the lower or upper vent (DFMT) individually or somewhat a combination of both. I use the lower vent as a course/medium adjustment and the dome to fine adjust.

Here is a beginning point and your settings may be different but in the end I would expect pretty close to these. Here is a link to a Visual Guide to Vent Settings

You will hear a lot about clear smoke, lighting the lump, here is a page with links to some useful information for egger's. Tips, Tricks and Useful Links

Getting your egg up to temperature will somewhat depend on how you light your lump.

Using starter cube(s), paper towel & oil, alcohol, will create a false temperature spike. That is, the heat will increase quickly from the flame of the starting material. It isn't uncommon to reach temperatures of 750°. However, there will also be a quick retreat of the temperature and it isn't uncommon to see that first low reach as low as 200°.

You will then see a slower increase in temperature, this increase is the lump burning. This is when you begin your course and then more refined vent adjustments. Don't adjust both vents at the same time.

Is you use MAPP, chimney lighter or weed burner, the initial spike is when you need to be adjusting the vents.

As you have already seen it is harder to get the temperature down on an egg than it is to get it to a higher temperature.

Again, welcome to the forum.

GG

I'm not an eggspert at all but to me that's the hardest thing to learn. Once you get the temp control down it's so much easier. It's a nightmare chasing temps. My first cook was the same thing that happened to you. I wasn't watching and temp went high. Took a long time to come down. Don't start cooking until you've maintain the temp that you want. Put your food on and don't touch the vents, it'll come back up. Enjoy