Ok how do I cut fabric straight??

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Sorry if this is a dumb question but I'm fairly new to sewing and seem to have an issue with cutting fabric straight, into a square or rectangle and to the size I intend. It always seems to go slightly squewiff. I have a rotary cutter and metal ruler and cutting mat thingy with a grid on it.

I'm hoping someone can tell me where I'm going wrong and maybe has a foolproof way of doing it that I could adopt!!

Practice- practice practice and a solid ruler with a metal edge.
Some quilting tools have a groove that you can sort of pair up with your cutter to keep you on track
But really
Sharp blade and practice practice practice

When I make curtains and blinds I use this

www.amazon.co.uk/Janome-Quilting-Ruler-24/dp/B0027IVPAM/ref=sr_1_1?ie=UTF8&qid=1424089842&sr=8-1&keywords=sewing+RULER

and this

www.amazon.co.uk/Stainless-Degree-0-30cm-0-20cm-Square/dp/B00JR3T6Q0/ref=sr_1_3?ie=UTF8&qid=1424089892&sr=8-3&keywords=metal+right+angle+ruler

along with an old fashioned yard stick for lengths.

I draw a line on my fabric with a biro or handwriting pen and then cut on the line with good dressmaking scissors.

I did buy a quilters ruler last month but I haven't used it yet

www.amazon.co.uk/gp/product/B0062IVY5E?psc=1&redirect=true&ref_=oh_aui_search_detailpage

it was recommended for small lengths by a friend. I have plans to make a quilt at some point along with too many other things.

Stuff that helps:
-pressing the fabric first
-measuring and drawing lines on first
If it's going wonky when you're using a rotary cutter it might mean you're not pressing down hard enough with the ruler and the fabric is stretching and slipping around as you cut.