Yes, London can be intimidating if you've never travelled to a big city before, but planning, as many previous posters have commented, is definitely the key.
There is lots of information on the Transport for London website with route planners, maps and so on. You can buy an Oyster card online and get it posted to you, so you don't have to queue at a ticket office or machine.
If you don't want to take public transport, you can get a black cab from whatever mainline station you arrive at. Taxi ranks are usually signposted within the station, and there are often 'marshalls' to assist.
There are lots of uniformed people in London (tube staff, police officers, PCSOs etc etc) who will all assist with any queries you have. Most Londoners will too, in spite of their reputation for unfriendliness.
Some simple precautions are always necessary wherever you travel to, never mind London - keep your handbag zipped up and under your arm, try not to use your smartphone out on the street etc, don't walk down dark alleyways, don't get in unlicensed minicabs.
Yes, if you're travelling at rush hour, people will push past you and walk quicker than you, but provided you're not standing stock-still in the middle of a narrow corridoor in a tube station, then just carry on at your own pace.
I'm sure you will be fine and confident, and have no trouble at all !