Pick a time and day when you know the roads won't be busy. For example, if you drive on a Saturday morning at around 4am, there'll hardly be any traffic and at that time, most A roads will be completely clear of any traffic at that time. This is probably your best course of action, I think it's the safest option for a beginner.
If you're nervous, you can drive at 60mph on the left lane.
Make sure you get some decent rest and sleep before you drive. Have a good meal before you set off.
There's nothing to be afraid of. For me, driving on motorways is less stressful than driving in busy city roads at peak traffic. If you're driving on the motorway late at night or early in the morning, you can have a very relaxed journey. It becomes very monotonous and boring if you're not doing anything, which is why I'm always listening to my favourite music while I drive.
Use a sat nav so that you know exactly which exits you need to take, slip roads, roundabouts, etc. And it tells you which lane you need to be in, it removes a lot of uncertainty and makes your journey much easier. For me, it's essential. It allows you to plan ahead.
Again, just to repeat...the best time to drive is very early in the morning on a weekend, the earlier the better.