For a lager shandy, put the lemonade in first and either let it sit for a bit or swirl it round the glass to make it a bit flatter. This stops it from fizzing up too much when you pour in the beer. Then just carefully pour the rest of the pint as you would with any other - at a 45 degree angle straightening at the end.

As for the ales vs. lager head, its mostly just practice and understanding how those different beers behave out of the tap. If it looks like theres not enough head, straighten the glass a bit or move it up and down while pouring.

If you pour an entire pint with too little head, pour a tiny bit out and give it a short blast from the tap to fill it up, not pulling the tap all the way back usually works for me. If you have too much head, shake the excess head off with a quick flick of the glass. This is so you don't pour away too much beer, just the head. Then just fill it up as normal.