Getting to Heathrow from Leceister Square

It depends on what time your flight leaves and whether or not you can manage your luggage down steps. The tube starts running shortly before 6am. It takes about an hour to get from Leicester Square to Heathrow. You will encounter some steps entering the tube station. And depending on what time you depart, you could hit rush hour which would mean the tube might be very crowded. A prebooked car service is another option. Just Airports is one that I frequently see recommended, but I have not personally used.

Luggage is manageable. What is the easiest way to get there using the tube. Flight leaves at 10:00 a.m.

It is a direct route. You take the Piccadilly line from Leicester Square to Heathrow.

The Tube will take 50 minutes plus some walking into the terminal. For an international flight you should be checking in at least 3 hours prior to scheduled departure. The Tube runs every 10 minutes around 6am, at xx:x9. If you need to be there at 7 you need to be on the tube before 6.

Flying to Germany. Do I still need 3 hours?

Possibly not, as it is internal, however if you are going 2 days before the Olympics there may be all sorts of reasons why things would go more slowly than normal. If it were I, I'd still get they with much spare time. You can always have a breakfast when you get checked in...

Thank you Nigel. I just noticed my flight leaves at 8:55 a.m.. So I guess the smart thing to do would be to catch the 1st train out at 6:00 on the Picadilly line to Heathrow. Does that sound reasonable?

You can get an earlier start from Paddington on either the Heathrow Express or Connect. https://www.heathrowexpress.com/timetable-schedule/london-heathrow https://www.heathrowconnect.com/content.asp?SID={43AB5F11-73FC-471C-AA45-1015A70476F4}&pageid=73 I have posted the timetables for both above--the Connect is cheaper than the Express but operates less frequently
The first Connect looks like it is at 0442 and the first Express is at 0530. Also you don't mention which terminal you are going to as this makes a difference. Check your airline website as well for what their checkin requirements are and realize that Heathrow at the best of times is as very busy place.

The problem with flying out of Heathrow is the (usual) extremely long lines to get through security to the departure longe. Whether you are flying to USA or Germany you still hve to go through the same security gate. I would still alow 3 hours

I am flying out of Terminal 5 if that makes a difference