Removing unwanted paint can range from easy to moderately difficult depending on the area that is being targeted. Water based paints are the easiest to tackle while oil based paints and varnishes can take a bit of work to completely remove. The method chosen will largely depend on the tools that you have on hand.
Typically you may need, sand paper, a paint scraper, a soap solution, thinner or a strong chemical paint or varnish removal solution. Before using the last option consider the more cost effective methods first.
To remove small spots of paint from areas that are not particularly porous or easily damaged consider using a paint scraper. A paint scraper can get beneath the layer of paint and lift it so that it comes off completely. If the paint has adhered too closely to the surface then consider using soap and water to loosen it or rub it with sand paper. If the paint is an oil based paint dab a cloth with a fair amount of lacquer thinner and rub the area until the paint has disappeared.
For large surfaces especially those that will be repainted, a chemical paint or varnish remover may be essential. Simply follow the instructions according to the manufacturer. Typically the surface will be inundated with the remover and left for a few minutes or more depending on the solution. After this the area can then be washed off with soap and water or easily rubbed off with a soft cloth or sand paper. For more ingrained layers of paint the use of a paint scraper in addition to the paint remover may be vital.
When removing the paint be careful not to inflict too much damage to the surface beneath. If some damage does occur especially on wood consider using a masonry repair solution or an all purpose joint compound to help smooth and correct these minor blemishes.
Tips and Tricks
Try to avoid using a paint scraper on less robust wood finishes. A medium grained sandpaper is best used instead to first lift the paint followed by a fine grained piece to smooth the area.