Its best to start learning boardslides on small rails, either you may have one if you dont your local park will probably have small rails in it. The boardslide consists of you ollie-ing on to the rail and landing on the rail at 90 degrees so your board is sliding and not your trucks. Approach the rail in yopur ollie position. When your approach the rail, ollie just high enough to get onto the rail. In mid air turn your body and board 90 degrees to get the board sliding. Your feet should be over the trucks bolts to keep the board sliding centrally on the rail. I have also been told by other skaters that they find it easier to boardslide when they stiffen their front leg while sliding. When approching the end of the rail just apply a little weight to your back foot and 90 off the rail
The noseslide is another easy slide although it can be liable to messing up when trying e.g sometimes you may ollie too high by accident and end up in a boardslide and fall off etc. The noseslide does not require a high ollie and I started to learn them on ledges about 2 and a half curbs high but 2 curbs high will do. Approach the ledge with your feet in the ollie position, maybe with your front foot a little more towards the nose than usual. Pop an ollie just high enough for the rail to keep your sliding momentum and shift all your weight to your front foot while truning 90 degrees to get your nose on the ledge. If you are having trouble sliding far enough lean back a little bit while ollie-ing into the noseslide. To get off just shift your weight to your back foot and 90 off the ledge.