The Russian verb has 3 tenses (present, past, future) and 2 aspects (работать — поработать, думать — подумать, рисовать — нарисовать). Aspects describe different qualities of an action—it is either acting (the process of doing something—Imperfective), the result of an action (after someone has finished doing something—Perfective) or a short time duration\periodicity (Perfective). As you see there can be built different perfective forms of one verb using different means, first of all - prefixes. The prefix по- usually means perfective with the meaning short time duration or one time action whereas prefixes c- на- у- за- ...mean the result. For example the verb to draw – рисовать (Imperfective) порисовать (Perfective for short time period), нарисовать (Perfective for the result - having completed the work). Many verbs don't have these two options, they have either perfective aspect of a result or of a time shortness. In the end it totally depends on the actual meaning of the verb.
However using the Imperfective or different forms of Perfective (if there are two) depends only on the intention of the speaker to emphasize different aspects of action in his or her speech.
We change the Imperfective aspect of the verb according to person (I, you he, we, you-plural, they).
Я работаю здесь = I work here \ I'm working here
Here there are two variants. We can use both aspects replacing the ть-ending with л.
Я работал здесь — I worked here many times\ I worked here (long time) \I was working here. (Imperfective)
Я поработал здесь — I worked here (a time period that I estimate to be short or one time). (Perfective: shortness)
Я думал о тебе — I thought about you many times\ I thought about you (long time period) \I was thinking about you. (Imperfective)
Я подумал о тебе — I thought about you shortly or one time.(Perfective: shortness)
Я делал это — I did it\ I was doing it (Imperfective)
Я поделал это — I did it shortly (Perfective: shortness)
Я сделал это — I have done it\ I had done it = I completed it (Perfective: result)
The same as with the past you can use both variants. If you change the Perfective aspect according to person you will have the Future tense that expresses that you will have done\completed something or that the action will last a short time period or will occur only once.
If you want to emphasize the process or that the action will have been done for a long time period or will occur many times in the future you should use the imperfective aspect putting an additional verb ( Я буду, ты будешь, он будет, мы будем, вы будете, они будут).
For example the verb рисовать\нарисовать