Assigning
Just because someone can do the task does not mean they should be assigned to do the task. Another resource can do the task however it might take them longer. If you build a plan and assign the 'best person' to do the job quickly to the majority of tasks then the project timeline will extend.
Begin by adding resources in the "Resource Sheet". Here you can not only add the resource name but also their rate, calendar and other details. Of course you can add resources from the "Gantt" view by simply typing in the resource name, however if you know who your resources are then you can add them all at once and then select them from the "Gantt" view drop down list. Use the 'resource sheet' to define resource levels in terms of Senior, Intermediate, or Junior. Then divide the work up. Maybe assign the most complicated work to the senior person. You can shorten your timeline by assigning work to resources intentionally choosing resources based on their skill level. This technique will help you avoid over allocating scarce resources to the majority of tasks.
Setting Yourself up for Success
When planning your project consider the work breakdown as well as the resource allocation. In most environments resources are limited so you will need to plan when resources can perform the task. Not only do you need to plan but you need to consider if the resource is working 100% on the task or if the resource is also working on other tasks during the timeline. In MS Project 'resource units' are how we define the percent of time a resource is working on a task. When you type a resource name next to a task if you do not see brackets "[]" with a percent complete in them, for example "[25%]", then the resource is allocated 100% on that task during the timeline (start and finish dates). If your resource really does need to work on the task 100% then you can use the predecessor field to link the task IDs based on when the resource availability. Yes, a task may have a dependency on another task's physical work as well as a resource dependency.
Estimates
Estimating requires previous knowledge, if you have done it before, or a crystal ball for gathering guesstimates.
A senior can complete a task sooner than an intermediate or junior resource. Scheduling tasks requires us to consider the resource's skill and estimated time to complete a task.
Just as a business stores information a project can store information and their level of expertise. This information determines how long it will take to complete the task. If you have no history from past experience with the resource's capability to provide accurate estimates then plan slack into the schedule since you are working with unknowns.