It looks like there is broad information about this over the Internet: http://www.gardeningknowhow.com/ornamental/shrubs/oleander/oleander-growing-tips.htm
Even in the garden, oleander shrubs require minimal care. Although the shrubs are drought-tolerant, they look their best when they are watered during dry spells. However, take care not to over water them. Yellowing leaves indicate that the plant is getting too much water. If the soil is poor, feed the plant lightly with a balanced fertilizer during its first spring. Once established, oleander shrubs don’t require routine fertilization.
Water newly planted oleanders regularly and deeply to establish a deep, wide-spreading root system.
Oleanders are fast-growing and need annual trimming to shape the plant and to keep it to the desired size.
Your plant looks as if it could do with a good prune, it's got a bit 'staring', that is, too much stem and not enough leaf. I grow these in London, UK, outdoors in sheltered spots, and the one I saw last week is now so bushy, thick with leaves and large, despite a good prune last year, it could do with pruning now - but it's about to flower. It's in this condition despite technically being too tender for outdoors in the UK, and having come through what passed for winter this year, so it'll be twice the size it is now in 3 months. I don't feed it at all, nor give it compost - but it's planted with its roots in shade, behind a low wall, and its upper parts in sun, which means it's probably always got access to moisture deep in the soil under the wall and paving. I'm guessing yours probably could do with a lot more water from late spring throughout summer to get good, bushy, green growth and more bouts of flowering. When pruning, don't get the sap on exposed skin - some people have a strong dermatological reaction to it. If you decide to prune, you can either do it now and sacrifice the early flowers, or wait till the usual, early crop of flowers is over.
In response to your comment, I almost mentioned the fact that your plant appears to be a 'standard' but I find many people don't understand the term even in the UK, and it confuses the issue! So what you're describing as 'a long step' means you have a straight stem with all the growth coming from the top, often grafted at that point, on the top of the stem, but sometimes a plant is simply trained to grow that way, and that is called a standard. A half standard is similar, but has a shorter stem. I can't see from the pic whether there's a graft at the top of the stem, but either way, you can't cut back past the top of that stem, so shorten all the growth down to 5 or 6 inches if you want, always cutting just above a leaf node, but no lower than that. This might encourage more unwanted growth elsewhere because yes, when you have a 'standard', you do remove any growth coming from the ground or off the long straight stem, to keep the form and shape. Even so, if you rub out or remove the unwanted growth as soon as it appears, the topgrowth should still grow thicker and bushier.