Sorry if its already been posted, but where is the Alien crash site?? I have heard that its up near Minefield!!

http://fallout.wikia.com/wiki/Alien_crash_site

It's a little North of MDPL-13 Power Station which is just North of Minefield. Just go straight North from the power station and when you go over some rocks and start getting irradiated, you should see the crash.

What about the Firelance, is it totally random!! i just completed the main quest in just over 17 hours, but i still have a save file from the start of the last mission, so i now have to install Broken Steel 2morro to carry on, so there is still time for it to happen, how will I know if the encounter has happened!!

Firelance is a random encounter that can only happen at one of the http://fallout.wikia.com/wiki/Fallout_3_random_encounters#Locations. Chances are you haven't triggered the event yet. It's entirely possible to visit every random encounter location in the game and never even get the Firelance event. If you're walking along, hear a Kaboom and see a flash streak across the sky, start looking for glowing Alien Power Cells. Dogmeat is very helpful in retrieving both the weapon and ammo once the event has happened.

I still havent got dogmeat yet, where can I get him please!!

He's inside the Scrapyard. It's near Minefield too.

Walk up the eastern edge of the Scrapyard. You should see him chewing up a bunch of raiders who just killed his previous master. After the carnage ends, whether you help him or not, he will approach you.